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

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)

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