Add backuptool support to the releasetools system

Pulled from CyanogenMod 10.1

Change-Id: Iffd9a45865011ad9108508d31f79e53d609dd6f4
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: Icd10de924f2b88168912c9575174cebf6b0104ba