"Steal" single value variables in rbc inheritance

This behavior of "stealing" values from parent makefiles is needed to
match make. We already had similar behavior for list variables via
the __move_items function, but were missing it for single value
variables.

Test: ./out/rbcrun -mode rbc ./build/make/tests/run.rbc
Change-Id: Ib320b9b1cce0224184f585c7a391be1b5353b440