Optionally dump RBC variables at the end of config.mk

config.mk represents essentially the entire product/board
configuration. In order to develop a "quick" rbc regression
test, dump all the make variables at the end of config.mk.

We can then compare these variable dumps instead of ninja
files, because the ninja files take much longer to generate.

Bug: 229132189
Test: Manually
Change-Id: I4e8371be446b7e511aba22dff58530a6d9ff072f