[LEDE-DEV] RFC: WIP: Adding support to automate generating UEFI bootable x86_64 and x86 images

[Alive 4ever]

I am currently working on adding support for LEDE build system to
generate UEFI bootable images.

What I've done in this WIP:
 [*] Adding grub2-efi-amd64 and grub2-efi-ia32 packages.
 [*] Adding new targets under x86: 64-efi and efi, which can be selected
     via build's menuconfig.
 [*] Modifying config/Config-images.in to select correct grub-efi
     package for x86_64_efi and x86_efi targets.
 [*] Modifying target/linux/x86/images/Makefile to create UEFI bootable
     LEDE images.

What's working:
 [*] UEFI bootable ext4 image

What doesn't work:
 [ ] UEFI bootable squashfs image
 [ ] GPT disk image (ptgen currently doesn't support gpt partitions)

Not yet tested
 [ ] RAMDISK image
 [ ] ISO image

Bugs I encountered:
 - Flickering screen during GRUB menu selection display (under EDK II
   firmware v 2.60)

My WIP tree can be found at
https://github.com/alive4ever/source/tree/x86_64-efi-automation

Please check the tree and give me review so that it's eligible for
inclusion on LEDE upstream.