Allow variable-prefixed include statements in mk2rbc

mk2rbc was already searching the whole android tree
for Makefiles, so allowing variable-prefixed include
statements doesn't affect performance on the file searching
front. On the generated code front, there's already a cap
of 150 potentially-included makefiles that prevents the
performance from getting too bad, and it can be lowered
if necessary.

Bug: 213508006
Test: go test
Change-Id: I3a4e81acb3d97bee08ac3dbe63052a274acf5793