sconfig: check whether component directory actually exists

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>