Revert "Revert "ota: Make sure we don't install on top of an incompatible system""

This should be fixed now.

This reverts commit 5875e3437d368411b986de70d3a844b27d78531f.

Change-Id: Ie102ff6ecb08bff80c83654a490fd18248d23086
Merge pull request #24 from CyanogenMod/cm-11.0

Upstream