pkg/libcoap: Add in libcoap CoAP library support

Includes a CoAP client and CoAP server example.

examples/libcoap-client/Kconfig

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+if USEMODULE_LIBCOAP
+config LIBCOAP_CLIENT_URI
+    string "CoAP URI to connect to"
+    default "coap://[fe80::405:5aff:fe15:9b7f]/.well-known/core"
+config LIBCOAP_USE_PSK
+    string "Secret to use for PSK communications"
+    default "secretPSK"
+    depends on USEMODULE_TINYDTLS
+config LIBCOAP_USE_PSK_ID
+    string "User ID to use for PSK communications"
+    default "user_abc"
+    depends on USEMODULE_TINYDTLS
+config LIBCOAP_SERVER_SUPPORT
+    bool "Set to y if server support is required"
+    default n
+config LIBCOAP_CLIENT_SUPPORT
+    bool "Set to y if client support is required"
+    default y
+endif # USEMODULE_LIBCOAP

examples/libcoap-client/Makefile

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+# name of your application
+APPLICATION = libcoap-client
+
+# If no BOARD is found in the environment, use this default:
+BOARD ?= native
+
+# This has to be the absolute path to the RIOT base directory:
+RIOTBASE ?= $(CURDIR)/../..
+
+# Include packages that pull up and auto-init the link layer.
+# NOTE: 6LoWPAN will be included if IEEE802.15.4 devices are present
+USEMODULE += netdev_default
+USEMODULE += auto_init_gnrc_netif
+
+# Activate ICMPv6 error messages
+USEMODULE += gnrc_icmpv6_error
+
+# Specify the mandatory networking module for a IPv6 routing node
+USEMODULE += gnrc_ipv6_router_default
+
+# Add a routing protocol
+USEMODULE += gnrc_rpl
+USEMODULE += auto_init_gnrc_rpl
+
+# Additional networking modules that can be dropped if not needed
+USEMODULE += gnrc_icmpv6_echo
+
+# Specify the mandatory networking modules for IPv6 and UDP
+USEMODULE += gnrc_ipv6_default
+USEMODULE += memarray
+USEMODULE += ipv4_addr
+
+# a cryptographically secure implementation of PRNG is needed for tinydtls
+# Uncomment the following 3 lines for tinydtls support
+CFLAGS += -DWITH_RIOT_SOCK
+USEPKG += tinydtls
+USEMODULE += prng_sha1prng
+
+# libcoap support
+USEPKG += libcoap
+
+# Configure if DNS is required
+# USEMODULE += sock_dns
+
+# Support 64 bit ticks
+USEMODULE += ztimer64_xtimer_compat
+
+# Add also the shell, some shell commands
+USEMODULE += shell
+USEMODULE += shell_cmds_default
+USEMODULE += ps
+USEMODULE += netstats_l2
+USEMODULE += netstats_ipv6
+USEMODULE += netstats_rpl
+
+# libcoap needs some space
+CFLAGS += -DTHREAD_STACKSIZE_MAIN=\(3*THREAD_STACKSIZE_DEFAULT\)
+
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+DEVELHELP ?= 1
+
+# Change this to 0 show compiler invocation lines by default:
+# QUIET ?= 1
+
+include $(RIOTBASE)/Makefile.include

examples/libcoap-client/Makefile.ci

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+BOARD_INSUFFICIENT_MEMORY := \
+    airfy-beacon \
+    b-l072z-lrwan1 \
+    blackpill-stm32f103c8 \
+    blackpill-stm32f103cb \
+    bluepill-stm32f030c8 \
+    bluepill-stm32f103c8 \
+    bluepill-stm32f103cb \
+    calliope-mini \
+    cc2650-launchpad \
+    cc2650stk \
+    hifive1 \
+    hifive1b \
+    i-nucleo-lrwan1 \
+    im880b \
+    lsn50 \
+    maple-mini \
+    microbit \
+    nrf51dk \
+    nrf51dongle \
+    nrf6310 \
+    nucleo-f030r8 \
+    nucleo-f031k6 \
+    nucleo-f042k6 \
+    nucleo-f070rb \
+    nucleo-f072rb \
+    nucleo-f103rb \
+    nucleo-f302r8 \
+    nucleo-f303k8 \
+    nucleo-f334r8 \
+    nucleo-l011k4 \
+    nucleo-l031k6 \
+    nucleo-l053r8 \
+    nucleo-l073rz \
+    opencm904 \
+    samd10-xmini \
+    saml10-xpro \
+    saml11-xpro \
+    slstk3400a \
+    spark-core \
+    stk3200 \
+    stm32f030f4-demo \
+    stm32f0discovery \
+    stm32f7508-dk \
+    stm32g0316-disco \
+    stm32l0538-disco \
+    stm32mp157c-dk2 \
+    yunjia-nrf51822 \
+    #

examples/libcoap-client/README.md

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+# libcoap client example
+
+This example shows how to configure a client to use libcoap
+
+## Fast configuration (Between RIOT instances):
+
+Preparing the logical interfaces:
+
+    sudo ./../../dist/tools/tapsetup/tapsetup --create 2
+
+## Client invocation
+For the client:
+
+    PORT=tap0 make term
+    coapc coap://[ip6-address]/some/path
+
+The IP address to connect to needs to be that as returned by libcoap_server,
+or that of the tap0 interface, etc.
+
+## Handling the static memory allocation
+
+libcoap for RIOT is using the `sys/memarray` module and therefore there
+are certain limits. Said resources are defined in `libcoap/src/coap_mem.c`,
+but can be overwritten at compile time.

examples/libcoap-client/app.config

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+CONFIG_LIBCOAP_CLIENT_SUPPORT=y
+CONFIG_LIBCOAP_CLIENT_URI="coap://[fe80::405:5aff:fe15:9b7f]/.well-known/core"
+
+CONFIG_LIBCOAP_USE_PSK="secretPSK"
+CONFIG_LIBCOAP_USE_PSK_ID="user_abc"
+
+CONFIG_KCONFIG_USEPKG_LIBCOAP=y
+# Logging levels are defined in pkg/libcoap using Kconfig