efi: Move fdt helper into own file

We only support FDT files with EFI on arm and arm64 systems, not
on x86. So move the helper that finds a prepopulated FDT UUID
into its own file and only build it for architectures where it
also gets called.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/Makefile.core.def

@@ -212,8 +212,10 @@ kernel = {
 
   arm_efi = kern/arm/efi/init.c;
   arm_efi = kern/arm/efi/misc.c;
+  arm_efi = kern/efi/fdt.c;
 
   arm64_efi = kern/arm64/efi/init.c;
+  arm64_efi = kern/efi/fdt.c;
 
   i386_pc = kern/i386/pc/init.c;
   i386_pc = kern/i386/pc/mmap.c;