Modify fstab.<board-name> file instead of modifying *.rc files

* This fixes partitions like /firmware, which has to be always
  mounted. It won't work with devices which don't use mount_all
  init command, but all new devices do, I hope.
Revert "Modify fstab.<board-name> file instead of modifying *.rc files"

This reverts commit e8ad384867b838b6e0b7d4fb55144838ee5655eb.
nexus4