kconfig: provide an option for enforcing Kconfig settings

This commit introduces an internal FORCE_CONF_FILE CMake setting which
allows higher order build systems to generate a configuration file
which will always take precedence.

This means that in case a user tries to change any setting to be
different than the defined value in the FORCE_CONF_FILE provide file(s),
then a warning will be printed and the setting will be reset to the
value given in the FORCE_CONF_FILE file.

Example of such warning:
   <path>/.config.sysbuild:1: warning: BOOTLOADER_MCUBOOT
   (defined at Kconfig.zephyr:766) set more than once.
   Old value "n", new value "y".

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