Makefile.inc: export VARIANT_DIR as top-level variable

export VARIANT_DIR at the top level, rather than doing so multiple
times at the SoC / board level

Change-Id: If825701450c78289cb8cca731d589e12aafced11
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/17451
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/Makefile.inc b/Makefile.inc
index cb0b21c..a193158 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -38,7 +38,9 @@
 #######################################################################
 # Basic component discovery
 MAINBOARDDIR=$(call strip_quotes,$(CONFIG_MAINBOARD_DIR))
+VARIANT_DIR:=$(call strip_quotes,$(CONFIG_VARIANT_DIR))
 export MAINBOARDDIR
+export VARIANT_DIR
 
 ## Final build results, which CBFSTOOL uses to create the final
 ## rom image file, are placed under $(objcbfs).