buildsystem: check for coreboot toolchain by default

buildsystem: check for coreboot toolchain by default

Other toolchains just don't cut it.

Change-Id: I7a0bdf60d89b5166c9a22c9e9f3f326b28f777b8
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/4584
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>