gpt: added libgpt
init: added mount_gpt command

- supports mounting partitions from MMC block devices based on the GPT
  unique partition GUID
- arbitrarily searches /dev/block/mmcblk{0-7} and takes the first GUID
  match it finds for a given mount_gpt command
- modified init.rc to mount /system and /data from the protodroid
  u-boot eMMC partition GUIDs