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 branch 'cm-11.0' of https://github.com/CyanogenMod/android_device_samsung_galaxys2-common into kitkat