a3: Reorganize all the things

* Remove no longer needed extra.fstab
* General reorganisation
* Use build number of *CN firmware
* Fixes for TWRP: Thanks to Dr-Shadow
* Use LOCAL_PATH everywhere
* Add a postrecoveryboot script to fix a bug in TWRP.
* Move some configs to a dedicated folder
* Correct battery capacity value

Change-Id: I1568103c5613828a87055ab9298b300a958bc2d1