Add otapackage support for backuptool

Change-Id: I512554c579d444067cd3ccbb0e6946a5eb6bc964a

Modular backuptool.sh.  Executes backup and restore methods defined in arbitrary /system/addon.d/*.sh scripts.

* Copy backuptool.functions alongside backuptool.sh.
* Delete both from /system/bin as they are not useful there.

Patch Series
============
http://review.cyanogenmod.com/#change,13265
CyanogenMod/android_build
  * edify generator
http://review.cyanogenmod.com/#change,13266
CyanogenMod/android_system_core
  * permissions on /system/addon.d
http://review.cyanogenmod.com/#change,13267
CyanogenMod/android_vendor_cm
  * 50-cm.sh reference backup script
  * modular backuptool.sh
  * support backuptool.functions used by /system/addon.d/*.sh scripts

Change-Id: I26b4907d28f49c69627699d2accd2f0fa2d1b112

update ota_from_target_files to handle mounting/unmounting for backupscript

backupscript should not be mounting/unmounting itself as it makes other
scripts have unexpected results (such as modelid_cfg, which expects /system
to be mounted)

instead have the ota script handle the mounting/unmounting

Change-Id: I94511f4147c624d975cb3ecbeaa8b0e98f63437c

build: Don't run backuptool on GMS builds

Change-Id: I5dde27f9d16b88049171db9805221d92e67f3e5d