Implement run-on-boot scripts support

* Scripts placed in /sdcard/multirom/roms/*rom*/run-on-boot are ran
  on boot. They must have have .sh suffix.
  Passed arguments:
      $1: Path to busybox
      $2: Path to ROM folder (e.g. /realdata/media/multirom/roms/rom)

  Scripts _have_ to use supplied path to busybox to run any commands,
  as there are no busybox links installed.