grub-efi: Use a variable to specify built-in grub modules.

The previous behavior defines a static set of modules that are built
into the grub efi executable. This works fine for a limited set of boot
environments namely the standard linux/initrd. This patch conditionally
 assigns the same modules to a variable. This allows other meta layers
to add additional modules or completely override the defaults. The use
case driving this patch is the use of multiboot2 and related modules.

(From OE-Core rev: 597f8e0040ba3135220000b23767858c64b5c9b8)

(From OE-Core rev: 63ce29a48b608f34dd378285a54c24ee83a3a1c7)

Signed-off-by: Philip Tricca <flihp@twobit.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-bsp/grub/grub-efi_2.00.bb

@@ -70,12 +70,14 @@ do_install_class-native() {
 	install -m 755 grub-mkimage ${D}${bindir}
 }
 
+GRUB_BUILDIN ?= "boot linux ext2 fat serial part_msdos part_gpt normal efi_gop iso9660 search"
+
 do_deploy() {
 	# Search for the grub.cfg on the local boot media by using the
 	# built in cfg file provided via this recipe
 	grub-mkimage -c ../cfg -p /EFI/BOOT -d ./grub-core/ \
 	               -O ${GRUB_TARGET}-efi -o ./${GRUB_IMAGE} \
-	               boot linux ext2 fat serial part_msdos part_gpt normal efi_gop iso9660 search
+	               ${GRUB_BUILDIN}
 	install -m 644 ${B}/${GRUB_IMAGE} ${DEPLOYDIR}
 }