Provide mechanism to local additions to the build

site-local/ is an optional directory for local additions to the build.
If site-local/Makefile.inc exists it will be parsed and used.

Use it to define VGA option roms, splash screens, extra rules to the
tree...

Change-Id: I0c6ee43ffa40e6c3f193db081ab551ab75bc7478
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/212
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>
diff --git a/Makefile.inc b/Makefile.inc
index 167ab5b..4c87bd2 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -45,6 +45,8 @@
 
 subdirs-$(CONFIG_ARCH_X86) += src/pc80
 
+subdirs-y += site-local
+
 #######################################################################
 # Add source classes and their build options
 classes-y := ramstage romstage driver smm