Add simple support for Pico

[erlingrj]

This PR adds bare metal support for RP2040 and cousins by implementing the platform API in terms of the Pico SDK. To run an example, the PICO SDK must be installed and an environmental variable PICO_SDK_PATH must point to the install location.

I have successfully compiled and linked the example program, but I have not yet flashed it onto a board as I don't have a RP2040 laying around.

[github-actions]

You will find how your pull request compares to your target branch in terms of memory usage below.

Memory Report

action_test_c

	from	to	increase (%)
text	71446	71446	0.00
data	976	976	0.00
bss	480	480	0.00
total	72902	72902	0.00

delayed_conn_test_c

	from	to	increase (%)
text	76067	76067	0.00
data	968	968	0.00
bss	480	480	0.00
total	77515	77515	0.00

event_queue_test_c

	from	to	increase (%)
text	23478	23478	0.00
data	648	648	0.00
bss	320	320	0.00
total	24446	24446	0.00

physical_action_test_c

	from	to	increase (%)
text	72685	72685	0.00
data	977	977	0.00
bss	1920	1920	0.00
total	75582	75582	0.00

port_test_c

	from	to	increase (%)
text	75999	75999	0.00
data	968	968	0.00
bss	480	480	0.00
total	77447	77447	0.00

reaction_queue_test_c

	from	to	increase (%)
text	23155	23155	0.00
data	648	648	0.00
bss	320	320	0.00
total	24123	24123	0.00

shutdown_test_c

	from	to	increase (%)
text	67976	67976	0.00
data	976	976	0.00
bss	480	480	0.00
total	69432	69432	0.00

startup_test_c

	from	to	increase (%)
text	67093	67093	0.00
data	976	976	0.00
bss	480	480	0.00
total	68549	68549	0.00

timer_test_c

	from	to	increase (%)
text	67079	67079	0.00
data	968	968	0.00
bss	480	480	0.00
total	68527	68527	0.00

trigger_value_test_c

	from	to	increase (%)
text	19312	19312	0.00
data	648	648	0.00
bss	320	320	0.00
total	20280	20280	0.00

[github-actions]

Memory usage after merging this PR will be:

Memory Report

action_test_c

	from	to	increase (%)
text	71446	71446	0.00
data	976	976	0.00
bss	480	480	0.00
total	72902	72902	0.00

delayed_conn_test_c

	from	to	increase (%)
text	76067	76067	0.00
data	968	968	0.00
bss	480	480	0.00
total	77515	77515	0.00

event_queue_test_c

	from	to	increase (%)
text	23478	23478	0.00
data	648	648	0.00
bss	320	320	0.00
total	24446	24446	0.00

physical_action_test_c

	from	to	increase (%)
text	72685	72685	0.00
data	977	977	0.00
bss	1920	1920	0.00
total	75582	75582	0.00

port_test_c

	from	to	increase (%)
text	75999	75999	0.00
data	968	968	0.00
bss	480	480	0.00
total	77447	77447	0.00

reaction_queue_test_c

	from	to	increase (%)
text	23155	23155	0.00
data	648	648	0.00
bss	320	320	0.00
total	24123	24123	0.00

shutdown_test_c

	from	to	increase (%)
text	67976	67976	0.00
data	976	976	0.00
bss	480	480	0.00
total	69432	69432	0.00

startup_test_c

	from	to	increase (%)
text	67093	67093	0.00
data	976	976	0.00
bss	480	480	0.00
total	68549	68549	0.00

timer_test_c

	from	to	increase (%)
text	67079	67079	0.00
data	968	968	0.00
bss	480	480	0.00
total	68527	68527	0.00

trigger_value_test_c

	from	to	increase (%)
text	19312	19312	0.00
data	648	648	0.00
bss	320	320	0.00
total	20280	20280	0.00

[github-actions]

Coverage after merging pico-support into main will be

35.11%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
external/nanopb
pb_common.c	0%	0%	0%	0%	100, 100, 100, 100, 100, 102, 106, 110, 110, 110, 110, 110, 112, 116, 119, 122, 124, 126, 126, 126, 129–132, 14, 14, 14, 142–144, 15, 150–152, 152, 152, 152, 152, 154, 156, 158, 160–161, 163, 166, 168, 17, 171–172, 172, 172, 177, 18, 181, 184–185, 188, 190–192, 195, 197, 197, 197, 199, 20, 20, 20, 20, 20, 201, 201, 201, 203, 207, 210, 210, 210, 215, 22, 221, 224, 226, 226, 226, 229, 231, 231, 231–232, 232, 232, 235, 238, 238, 238, 24, 241–242, 246, 248, 248, 248, 25, 250, 254, 26, 260, 263, 265, 265, 265, 267, 269, 269, 269, 27, 272–273, 277, 28, 286–287, 29, 290, 292, 295, 297, 300, 302, 302, 302, 304, 306, 306, 306, 308, 308, 308, 308, 308, 310, 313, 313, 313, 313, 313, 315, 32, 320, 34, 36–41, 44, 46–48, 50–55, 58, 60–63, 65–70, 74, 74, 74, 77–78, 8, 82, 84, 84, 84, 86, 88, 88, 88–89, 89, 89–90, 90, 90, 93, 97
pb_decode.c	0%	0%	0%	0%	100, 1000–1001, 1003–1004, 1007, 1007, 1007, 1009, 1009, 1009, 1011, 1011, 1011–1012, 1012, 1012, 1016, 1016, 1016, 1022, 1022, 1022, 1024, 1024, 1024–1025, 1027, 1030, 1030, 1030, 1032, 1032, 1032, 1034, 1038, 1038, 1038, 104, 104, 104, 1042, 1042, 1042, 1042, 1042, 1045, 1045, 1045, 1047, 1047, 1047, 1049, 105, 105, 105, 1050, 1053, 1053, 1053, 1055, 1059, 1059, 1059, 1061, 1063, 1063, 1063–1064, 1066, 1066, 1066, 1069, 1074, 1074, 1074–1076, 108, 108, 108, 1082, 1082, 1082, 1082, 1082, 1084, 1084, 1084, 1089, 1089, 1089, 109, 109, 109, 1090, 1090, 1090, 1092, 1092, 1092, 1095–1097, 1100, 1103, 1103, 1103–1104, 1104, 1104, 1106–1107, 1110, 1110, 1110–1111, 1115, 1115, 1115–1116, 1116, 1116, 1118, 1118, 1118, 1123, 1125, 1125, 1125, 1129, 1129, 1129–1130, 1133, 1133, 1133, 1135, 1135, 1135–1136, 1136, 1136, 1140, 1140, 1140, 1142–1143, 1143, 1143, 1145, 1145, 1145, 115, 115, 115, 1151, 1154, 1158, 1158, 1158, 116, 1160, 1165, 1165, 1165–1166, 1168, 1170, 1170, 1170–1171, 1179, 118, 1182, 1186, 1193, 120, 125, 127, 127, 127–128, 128, 128, 131, 131, 131–132, 132, 132, 1336, 1341, 1346, 1349, 1349, 1349–1350, 1352–1353, 1356, 1359, 1359, 1359–1360, 1362, 1362, 1362–1363, 1365, 1367, 1370, 1377, 1377, 1377–1378, 138, 1382, 1389, 1393, 140, 1400, 1400, 1400–1401, 1405, 1416, 1420, 1422, 1425, 1427, 1427, 1427, 143, 1430, 1430, 1430–1431, 1434, 1434, 1434–1436, 1436, 1436–1438, 1438, 1438–1440, 1440, 1440–1441, 1443, 1443, 1443, 1445, 1445, 1445–1446, 1446, 1446, 1448, 1456, 1456, 1456, 1458, 1458, 1458–1459, 1463, 1463, 1463–1464, 1472, 1472, 1472–1473, 1475, 1479, 1479, 1479–1481, 1481, 1481–1483, 1483, 1483–1485, 1485, 1485–1486, 1488, 1488, 1488, 1490, 1490, 1490–1491, 1491, 1491, 1493, 1497, 1503, 1503, 1503–1504, 1506, 1506, 1506–1507, 1507, 1507, 1509–1510, 1510, 1510–1511, 1511, 1511, 1513, 1513, 1513, 1516, 1516, 1516, 1528, 1528, 1528–1529, 1529, 1529–1530, 1533–1534, 1537, 1541, 1543, 1543, 1543–1544, 1546, 1546, 1546–1547, 1547, 1547, 1550, 1552, 1552, 1552–1553,