abuild: silence make

make called within make prints 'Entering directory'
cruft which confuses the architecture support test.
Silence it.

Change-Id: I7ce7e0ff49e9317fe736ed80f5f18186d416ae63
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/7968
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)