[LEDE-DEV] Add support for native UEFI boot on x86_64

[Alive 4ever]

I am experimenting with UEFI boot for LEDE x86_64 image.

Currently, lede-x86_64-vmlinuz boots fine on UEFI - both on MBR and GPT
disk, by chainloading via grub or directly launching the kernel via EFI
shell.

There is no support in the kernel for EFI framebuffer, so a serial
interface is needed to interact with the system. I have reported this
on FS#515.

For EFI boot to work, FAT16/FAT32 formatted boot partition is needed.
It seems that the imagebuilder doesn't include a tool to create FAT
formatted partition (dosfstools).

GRUB should be called with argument '--target=x86_64-efi' in the image
generation phase. Important grub modules to include for EFI boots are
part_gpt (for gpt partitioned disk support), efi_gop (for EFI graphics
output protocol framebuffer), efi_uga (for EFI universal graphic adapter
framebuffer) in addition to GRUB2_MODULES in the image Makefile. I think
a new grub2-efi package is needed for this to work.

A new subtarget for x86_64 efi is probably needed. This new subtarget
will have a combined image with a FAT32/FAT16 EFI system partition and
another ext4 rootfs partition.