Build - ota: Make sure we don't install on top of an incompatible system

If the system has data present, make sure said data doesn't depend
on a system signature different from our own. If we can't find
our platform key, abort the installation

Change-Id: I0106fcc47b256dc71ae8dc9b8c4c9ef05462e0dc
Signed-off-by: STELIX <ssspinni@gmail.com>