libpayload: Allow using CBFS functions on images in RAM

Two new functions allow switching the CBFS functions from using RAM
or ROM, with ROM as default.

Change-Id: I04d67ad622d25c5728ae9a63f5b8a3dc9bbacce6
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/550
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/cbfs.h

@@ -1,2 +1,4 @@
 #include <arch/types.h>
 #include "cbfs_core.h"
+void setup_cbfs_from_ram(void* start, uint32_t size);
+void setup_cbfs_from_flash(void);