Several variables must be set by the time boilerplate.cmake is run.
This includes BOARD, SHIELD, CONF_FILE, and DTC_OVERLAY_FILE.
It's not always clear to users that this is the case, and we sometimes
see attempts to modify these variables in app CMakeLists.txt files
after find_package(Zephyr ...) lines, where they will have no effect.
This causes confusion.
Add a new extension function, zephyr_boilerplate_watch(), which traps
attempts to set these variables and prints a noisy warning. Use it
from boilerplate.cmake on the variables mentioned above.
This will hopefully make it clearer at build time why the changes are
being ignored.
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
0629e698af