doc: clarify how to set $FOO_ROOT in CMakeLists.txt
doc: clarify how to set $FOO_ROOT in CMakeLists.txt

I received feedback that setting e.g. BOARD_ROOT in CMakeLists.txt is
not quite well specified; in particular, it's important to do so
before pulling in the boilerplate via find_package(). Clear that up.

Reported-by: Pete Skeggs <7623f482543c6b98f7c630b0491057372eca75e9@nordicsemi.no>
Signed-off-by: Martí Bolívar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

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