Rename VB_SOURCE to VBOOT_SOURCE for increased clarity

This renames the VB_SOURCE variable to VBOOT_SOURCE in the build system,
providing increased clarity about what it represents.

Since the submodule itself is called "vboot", it makes sense to use that
name in full instead of a very shortened (and confusing) version of it.

Change-Id: Ib343b6642363665ec1205134832498a59b7c4a26
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-on: https://review.coreboot.org/15824
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/util/futility/Makefile.inc b/util/futility/Makefile.inc
index d37be4b..782953e 100644
--- a/util/futility/Makefile.inc
+++ b/util/futility/Makefile.inc
@@ -2,7 +2,7 @@
 
 $(objutil)/futility/build/futility/futility:
 	@printf "    MAKE       $(subst $(objutil)/,,$(@))\n"
-	unset CFLAGS LDFLAGS; $(MAKE) -C $(VB_SOURCE) \
+	unset CFLAGS LDFLAGS; $(MAKE) -C $(VBOOT_SOURCE) \
 		BUILD=$(abspath $@/../..) \
 		CC="$(HOSTCC)" \
 		V=$(V) \