Add otapackage support for backuptool

Change-Id: I512554c579d444067cd3ccbb0e6946a5eb6bc964a
Allow override of device asserts, including multi-device support.

Set in board file with TARGET_OTA_ASSERT_DEVICE.
(cherry-picked from commit 0f452f21fc9323b9d1fe746161761cf40aaa5030)

Change-Id: I3d06bdc0e3e26bde0c0e646accd050364f9713b9
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