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

Tips & Tricks
=============
* 50-cm.sh contains only a reference implementation.  You may customize the methods however you wish.
  For example, 20-foobar.sh pre-backup can use a loop with conditionals to generate a dynamic backup list in
  /tmp/foobar_file_list which is later printed by list_files() so the backup method will act on those files.
* Optional methods pre-backup, post-backup, pre-restore, or post-restore may be defined for special purposes.
* Inject new files into /tmp/addon.d/ prior to backuptool.sh backup if you want to act during the current CM upgrade.
* Delete files from /tmp/addon.d/ during post-restore if you want to permanently remove files from /system/addon.d/
  Addons may use this approach to run a script only once.
* Scripts run in sort -n order.  Prefix with numbers 00 through 99 if want to run in a particular order.
* You can have two separate scripts, implementing only backup in one, and only restore in the other with a different
  number prefix of each.  This allows even greater control the backup/restore order even further.
* You could use pre-backup to generate a one-time use backup script in /tmp/addon.d/ that deletes itself in
  post-restore.

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: Ifd5eaf9dcfd68d92e5043c21d1bae1dc0ad54860