staging: greybus: Add drivers/staging/greybus to the build

This adds a proper Kconfig file for drivers/staging/greybus and fixes up
the Makefile to work correctly within the kernel build system (modules
depend on the .config options, etc.)

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
diff --git a/drivers/staging/Makefile b/drivers/staging/Makefile
index 9f6009d..210fcea 100644
--- a/drivers/staging/Makefile
+++ b/drivers/staging/Makefile
@@ -41,3 +41,4 @@
 obj-$(CONFIG_MOST)		+= most/
 obj-$(CONFIG_ISDN_I4L)		+= i4l/
 obj-$(CONFIG_KS7010)		+= ks7010/
+obj-$(CONFIG_GREYBUS)		+= greybus/