This patch adds a new image class 'image_dd', by default
it has an empty boot partition. image_dd_efi extends this
class to support Intel NUC board. The layout of a generated
image is a prerequisite for OSTree integarion.

Image generated by 'hddimg' produces rootfs.img (ext3
filesystem file) and then "live" boots it from initramfs
by mounting rootfs.img via loop device. For OTA integration
we need to access contents of rootfs.img already from
boot loader.

The custom 'image_nuc_efi' layout allow for fully atomic
updates on EFI based system, by keeping GRUB-EFI binary
on a dedicated EFI system partition (ESP). OSTree currently
is not fully atomic on EFI systems, but with this setup
we use GRUB-BIOS code path in OSTree, which is atomic.
After EFI firmware has loaded the GRUB-EFI binary, everything
else is done on rootfs parition.

One limitation from the above scenario is that you would
need to update /EFI/BOOT/bootx64.efi (GRUB-EFI) binary
manually. Not sure how common it is to update a boot loader
binary on a deployed system, but its not impossible.

Change-Id: Ibf2840aecd548000372131c4ded5cffa11ff1b0f
Reviewed-by: Teemu Holappa <teemu.holappa@theqtcompany.com>