scripts: add west build, flash, and debug commands

West now supports a mechanism for extension commands. Use it to move
the command implementations that are tightly coupled with boards and
the zephyr build system back into the Zephyr repository.

This patch doesn't include test cases. Those will be moved over in a
subsequent patch.

Signed-off-by: Marti Bolivar <marti@foundries.io>
26 files changed