lib/prog_loaders: Move argument selection into selfload

Set the payload argument in selfload, as other (non self) payloads, are
going to set a different argument.

Change-Id: I994f604fc4501e0e3b00165819f796b1b8275d8c
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/25861
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/program_loading.h

@@ -202,10 +202,10 @@ void mirror_payload(struct prog *payload);
 /*
  * Set check_regions to true to check that the payload targets usable memory.
  * With this flag set, if it does not, the load will fail and this function
- * will return NULL.
+ * will return false. On successful payload loading this functions return true.
  *
  * Defined in src/lib/selfboot.c
  */
-void *selfload(struct prog *payload, bool check_regions);
+bool selfload(struct prog *payload, bool check_regions);
 
 #endif /* PROGRAM_LOADING_H */