doc: supporting module Kconfig code in module ext root
doc: supporting module Kconfig code in module ext root

Fixes: #31486

Sourcing `cmake/extensions.cmake` to have Zephyr functions available
when including Zephyr CMake files into doc build system.

This is a follow up on #30904 by supporting generated Kconfig files
that might contain lines as `osource "$(ZEPHYR_<MODULE_NAME>_KCONFIG)"`
to be properly parsed for doc building.

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

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