add support for overriding build.prop settings.

Change-Id: Ie4224d608b7bcb0c2087bd77bc954f8f5f9b6495
Add ro.screwd.device to build.prop

Based on CM's commit here:
https://github.com/CyanogenMod/android_build/commit/0e4e65fe104963de041579f3dc141357e3fa2afb