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