cmake: move the BOARD variable watch below deprecated board handling
cmake: move the BOARD variable watch below deprecated board handling

Fixes: #33138

Board alias and deprecated board handling is setting the BOARD variable
after zephyr_boilerplate_watch(BOARD) has been executed.

This results in the board changed warning.

This commit moves the zephyr_boilerplate_watch(BOARD) code below the
board alias and deprecation handling.

Signed-off-by: Torsten Rasmussen <a7c5b1d5a1f41fce6edfa3f7c2392ae0c59deafe@nordicsemi.no>

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