system-image: add PGP keys for system-image.tasemnice.eu server

Change-Id: I063578d39a4498c37642f8e0f493c015685ac01d
system-image: store base_url file for OTA updates

Change-Id: I969a304f5ca478918f04a5ac25ab7b13b4613eb8
Merge branch 'master' into merge-27