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 <peter.skeggs@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>