evita: Add USB PROXY device/use cases

Change-Id: I454b62f23163c54b032c64365f8dc10bb99cc021
evita: Use by-name partitions in fstab

* Also change encryption to use the 'extra' partition.
* Also remove recovery specific fstab

Change-Id: Ibd29b172b7560b628aab0e8cec02930b0fdbd836
evita: default to MTP USB mode

You can't default to ADB since user is required to enable USB debugging
before adb kicks in. UMS was deprecated to help save users from
themselves (unclean unmounts and apps left wondering what happened to
their storage volume). We should at least default to MTP, even if users
will opt to change this later.

Change-Id: I534091a5fb64dc02c4bb31485714d990fb7c1931
evita: Create firmware symlinks at build time

Change-Id: I2afb2424a0ea719c9a6db17af9ebe4941b160a1c