efi_loader: Add GOP support

The EFI standard defines a simple boot protocol that an EFI payload can use
to access video output.

This patch adds support to expose exactly that one (and the mode already in
use) as possible graphical configuration to an EFI payload.

With this, I can successfully run grub2 with graphical output.

Signed-off-by: Alexander Graf <agraf@suse.de>

include/efi_loader.h

@@ -89,6 +89,8 @@ extern struct list_head efi_obj_list;
 
 /* Called by bootefi to make all disk storage accessible as EFI objects */
 int efi_disk_register(void);
+/* Called by bootefi to make GOP (graphical) interface available */
+int efi_gop_register(void);
 /*
  * Stub implementation for a protocol opener that just returns the handle as
  * interface