latona: fixes for the BML to MTD conversion

* Try to mount both /sdcard and /emmc and don't unmount them.
  In this way the zip can be flashed from both the locations
  and recovery can find the zip after the reboot.

* Check the existence of the zip file. This prevents loops in case
  the zip is not found.
Merge pull request #2 from sconosciuto/jellybean

latona: fixes for the BML to MTD conversion