cmake: make WATCH an optional argument to zephyr_check_cache()

Watching of specific Zephyr CMake build variables ensure users cannot
accidentally set important variables, such as BOARD, SHIELD, CONF_FILES
late in the build process.

This was also added to zephyr_check_cache() function.
However, the BOARD provided by the user may look as `<board>@<revision>`
and will be kept in the cache in that form.

However, the BOARD value from cache is spit into internal build
variables as: BOARD=<board> and BOARD_REVISION=<revision>.
However, this is done after `zephyr_check_cache()` which then triggers
the variable watch to trigger, and printing a warning when using board
revisions.

Therefore WATCH is now optional for `zephyr_check_cache()` and watch for
BOARD variable will be enabled when boilerplate has completed all board
handling.

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