scripts: add board revision help text for west build
scripts: add board revision help text for west build

west build supports specifying the board revision using @revision, but
it's not mentioned in the help text. This commit updates the help text
to describe how to specify the board revision.

Signed-off-by: Gregers Gram Rygg <8588df0a4b84cfe86b7b6d581b64504a273a5e94@nordicsemi.no>

zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr