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

prepare_sysroot function is splitted into mount_sysroot to mount system and use mkfs.ext4 from there (no need to increase the size of initramfs)