mkefidisk.sh: Allow using a loopback mounted file

mkefidisk.sh: Allow using a loopback mounted file

It should be possible to generate a disk to a file using a loopback
device with mkefidisk.sh, which is useful for booting simulators.  To
make this possible the partitions for the loop back need to work
similarly to the mmc devices.  The mkfs.vfat also requires and
additional argument to force it to write to something other then a
real disk.

Example:
  qemu-img create -f raw bigdisk 4G
  dev=`sudo losetup -f`
  sudo losetup $dev bigdisk
  mkefidisk.sh $dev tmp-eglibc/deploy/images/qemux86/core-image-minimal-qemux86.hddimg /dev/sda
  sudo losetup -d $dev

Note:
  Also a bug was fixed in the mkefidisk.sh where if the disk you are
  writing to initially has an invalid label the size of the first
  partition will be computed incorrectly.  For the simulator disk
  creation this is generally always the case, but this can happen with
  real hardware as well.

(From OE-Core rev: 254899824900f2e8c6a34d2ad1b8cbea91acb4ae)

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>