xcompile: document all the variables!

What's the exact difference between TARCH, TSUPP and TBFDARCHS? Fear no
more, it's documented.

Change-Id: I18717eb1e20b1c0a82a485d391de2794a77c59ae
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/13419
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)
1 file changed