Store the base64 release key in the OTA zips

This can be directly grepped in pre-existing package.xml tables
to make sure we're not trying to update to a differently signed
build

Change-Id: I7528a8e7c484ea9209cd665b9263328ae834586a
superficial renaming of the META/.../releasekey path

Change-Id: I51489b34bab6b5fd4fb139608234b51a1e57dcc4
tools: getb64key - print the base64 version of a PEM public key file

Change-Id: Ia94ff560c88dfe85c3fa55db5c8219aa0d3419ee