galaxys2: Handle boot.img properly in ota script

The OTA script may be run during a signing process and will not have
OUT variables defined. Use the common methods for getting the boot.img
to match standards.

Change-Id: I4eebf58ec828f18f62176a2862e5fdbb839cb658
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0