sconfig: check whether component directory actually exists

and add drivers/generic/generic back (empty), since it is used by many
devicetree.cb files.

Without this patch typos in component names in devicetree.cb cause
the component to be silently ignored.

Change-Id: I3cfca2725816f0cd7d72139ae53af815009e8ab4
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/270
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>
diff --git a/src/drivers/generic/Makefile.inc b/src/drivers/generic/Makefile.inc
index 0700a0c..08e6cc5 100644
--- a/src/drivers/generic/Makefile.inc
+++ b/src/drivers/generic/Makefile.inc
@@ -1 +1,2 @@
 subdirs-y += debug
+subdirs-y += generic