abuild: pass compiler configuration options to tool building step

This is required to run abuild parallely with clang
without the canonical coreboot toolchain installed.

Change-Id: Iea56d3f552d50ab6e762afa134091b0d8e38792c
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/7369
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>