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: I16572daf9464de326a9d0d126597cfbf03208f94
build: Store the jenkins build number in misc_info

If the jenkins build number is available, list it as build_number in
the misc_info.txt file.

Change-Id: I6500c6b53661f2904dff738d25d8a8d24a7dca80