recovery: support for more than 1 sdcard in USB mass storage

  * Attempts to share any device behind /sdcard, /emmc, and /external_sd
    (in that order)
  * New option in recovery.fstab: lun=<path to LUN file>
      - specifies path to LUN file for given mount point
      - used when sharing a device over USB mass storage
      - restriction: only applicable to /sdcard, /emmc and /external_sd
  * If not specified in recovery.fstab, searches for available LUN files in order:
      1. BOARD_UMS_LUNFILE (if defined)
      2. /sys/devices/platform/usb_mass_storage/lun%d/file
      3. /sys/class/android_usb/android0/f_mass_storage/lun/file
      4. /sys/class/android_usb/android0/f_mass_storage/lun_ex/file
      ** More can be added to this list for better auto detection
  * Searches for available LUN files using %d replacement
     - %d is expanded to 0 and 1 only

Change-Id: I09e91f9b6c1e2c5e96df8983b1132deeec6395b5