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