fix(coreos/config): Cleanup make.conf spaghetti, change target binhosts.

Replace ChromiumOS target binhosts with our own. The auto-generated
files have been stubbed out and configs for targets we don't currently
support have been removed. MARCH_TUNE is now defined here.

After this change target binhosts will defined here and
/etc/make.conf.board and overlay-amd64-generic/make.conf are no longer
used. The new setup_board only creates /etc/make.conf.board_setup.