ARMv7M: add arm cmd group

- Add arm cmd group to armv7m cmd chain.
 - arm cmd's now check the core type before running a cmd.
 - todo: add support for armv7m registers for reg cmd.

Signed-off-by: Spencer Oliver <ntfreak@users.sourceforge.net>

src/target/armv7m.c

@@ -877,6 +877,9 @@ static const struct command_registration armv7m_exec_command_handlers[] = {
 	COMMAND_REGISTRATION_DONE
 };
 const struct command_registration armv7m_command_handlers[] = {
+	{
+		.chain = arm_command_handlers,
+	},
 	{
 		.name = "dap",
 		.mode = COMMAND_EXEC,