Increase flexibility for serial devices in run-tool

Instead of using a fixed command like picocom in the run tool,
it's better to have the flexibility to set the whole command that
connects to the serial device. Therefore, it's possible for instance
to connect to a remote serial device exported via TCP/IP by using tools
like socat.

tool/run

@@ -266,7 +266,7 @@ set specs            [get_cmd_arg --specs            ""]
 set repositories     [get_cmd_arg --repositories     ""]
 set qemu_args        [get_cmd_arg --qemu-args        ""]
 set run_target       [get_cmd_arg --target       "qemu"]
-set serial_dev       [get_cmd_arg --serial-dev "/dev/ttyUSB0"]
+set serial_cmd       [get_cmd_arg --serial-cmd "picocom -b 115200 /dev/ttyUSB0"]
 
 
 
@@ -674,7 +674,7 @@ proc exit {{status 0}} {
 #
 proc spawn_serial { wait_for_re timeout_value} {
 	global spawn_id
-	global serial_dev
+	global serial_cmd
 	global run_target
 
 	if {$wait_for_re == "forever"} {
@@ -686,8 +686,8 @@ proc spawn_serial { wait_for_re timeout_value} {
 	if {[regexp "reset" $run_target]} {
 		power_plug_reset
 	}
-	
-	spawn picocom -b 115200 $serial_dev
+
+	eval spawn $serial_cmd
 	set serial_spawn_id $spawn_id
 	wait_for_output $wait_for_re $timeout_value $serial_spawn_id
 }