vendor: Dev signing keys: Set keys location with an env var

This gives us more flexibility without having to edit the source over
and over again.

[ AgentFabulous - POSP ]
- Refactor variable to incorporate build/make changes.
- Check var SIGNING_KEYS and sign based on basis of that.
- Final OTA package will still differ, but this will let us sideload
  files in outdir without much trouble.

Change-Id: Idf3ef4b56ff0fe73c009073d62c35eec35e2a016
Signed-off-by: Kshitij Gupta <kshitijgm@gmail.com>
Signed-off-by: SahilSonar <sss.sonar2003@gmail.com>
1 file changed