coreboot: unify infrastructure for loading payloads

A payload can be loaded either from a vboot region or from cbfs.
Provide a common place for choosing where the payload is loaded
from. Additionally, place the logic in the 'loaders' directory
similarly to the ramstage loader infrastructure.

Change-Id: I6b0034ea5ebd04a3d058151819ac77a126a6bfe2
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5296
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/lib/selfboot.c

@@ -32,6 +32,7 @@
 #if CONFIG_COLLECT_TIMESTAMPS
 #include <timestamp.h>
 #endif
+#include <payload_loader.h>
 
 /* Maximum physical address we can use for the coreboot bounce buffer. */
 #ifndef MAX_ADDR