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>

src/drivers/generic/Kconfig

@@ -1,2 +1,3 @@
 source src/drivers/generic/debug/Kconfig
+source src/drivers/generic/generic/Kconfig
 

src/drivers/generic/Makefile.inc

@@ -1 +1,2 @@
 subdirs-y += debug
+subdirs-y += generic

src/drivers/generic/generic/Kconfig

@@ -0,0 +1,2 @@
+config DRIVERS_GENERIC_GENERIC
+	bool

src/drivers/generic/generic/Makefile.inc

@@ -0,0 +1 @@
+# Empty