guid: Unify GUID types

There are 3 implementations of a GUID in GRUB. Replace them with
a common one, placed in types.h.

It uses the "packed" flavor of the GUID structs, the alignment attribute
is dropped, since it is not required.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/kern/efi/fdt.c

@@ -24,7 +24,7 @@ void *
 grub_efi_get_firmware_fdt (void)
 {
   grub_efi_configuration_table_t *tables;
-  static grub_efi_guid_t fdt_guid = GRUB_EFI_DEVICE_TREE_GUID;
+  static grub_guid_t fdt_guid = GRUB_EFI_DEVICE_TREE_GUID;
   void *firmware_fdt = NULL;
   unsigned int i;