Implement custom bootimg to merge recovery into boot ramdisk

Custom makefile does following:
* Moves recovery/root/init.rc to root/recovery.rc
* Moves recovery/root/default.prop to root/default.prop.recovery
* Merges the remaining contents of recovery/root/ to root/
* Avoids recovery.img generation (mka recoveryimage will result in
  boot.img being built).

This allows for a fully functional merged boot and recovery.

Change-Id: Ie397b8e7ce1b866ea5822793b80f23861deee528