abuild: Build boards in parallel if possible

Determine if xargs -P works. If yes, use that to build multiple
boards in parallel, instead of relying on make -j X, when doing
a full abuild run (instead of single boards).

make -j X isn't able to make use of several cores at various
serialization points in our build process, so this change results
in a >25% speed up for a full abuild run in my tests.

Change-Id: Id484a4211c84a3a24115278e0fbe92345f346596
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/409
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/Makefile.inc b/Makefile.inc
index 01fb41f..176ff67 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -298,3 +298,5 @@
 crossgcc-clean: clean-for-update
 	$(MAKE) -C util/crossgcc clean
+tools: $(objutil)/kconfig/conf $(objutil)/cbfstool/cbfstool $(objutil)/nvramtool/nvramtool $(objutil)/romcc/romcc $(objutil)/sconfig/sconfig