luv-live-image: A new live image for luv

Having a hard drive image to boot with qemu is fine and dandy but we
really need a bootable image that we can write to removable media and
run on real hardware, not least because that's the thing people will
expect to find on the project website.

bootimg.bbclass provides much of the functionality we need and already
contains code to build a FAT file system image from our rootfs.

The customization occurs in do_create_img() where we construct a
partition table and append the rootfs FAT file system. While scripts
already exist to build this type of image (scripts/contrib/mkefidisk.sh)
they explicitly disallow booting with an initramfs as the only file
system. The initramfs is integral to our boot procedure.

Tested on real UEFI hardware.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>