cbfs: expose init_backing_media()

I broke cbfs loading with commit 358901. As multiple
functions are being reused one needs to ensure there is
always a cbfs media object allocated on the stack and
initialized. Ya for no common writable globals.

TEST=Ran qemu-armv7. CBFS loading works again.

Change-Id: Ibd047af7dcd8575e6203651471079fc2042da282
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/8973
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Duncan Laurie <dlaurie@google.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: Aaron Durbin <adurbin@google.com>
Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>

src/include/cbfs.h

@@ -52,6 +52,7 @@
 
 #include <cbfs_core.h>
 
+int init_backing_media(struct cbfs_media **media, struct cbfs_media *backing);
 void *cbfs_load_optionrom(struct cbfs_media *media, uint16_t vendor,
 			  uint16_t device, void * dest);
 void *cbfs_load_stage(struct cbfs_media *media, const char *name);