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 <Torsten.Rasmussen@nordicsemi.no>