Collect generated rbc files under one folder

Mk2rbc takes a .mk file and produces a .rbc file under the
same path, but with a different stem. For example:
build/test/foo.mk becomes out/build/test/foo.rbc

This makes it difficult to see everything that mk2rbc has
generated in the out folder. Move the generated files to
out/rbc instead, so they have a common stem that is separate
from the rest of the build outputs.

Fixes: 202249430
Test: m RBC_PRODUCT_CONFIG=1 nothing
Change-Id: If3edba3feef9c2d3631244d533b997ef0b8b4e8b