liveusb: enable running OpenELEC from only one USB stick
possible syslinux menu with option for running installer or live system
new mount type FILE with optional size in MB for loopback file
  disk=FILE=storage.loop
  disk=FILE=storage.loop,100

dd and mkfs.ext4 are used from system (no need to increase the size of initramfs)
Merge pull request #2082 from vpeter4/liveusb

liveusb: enable running OpenELEC from only one USB stick