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>
diff --git a/config/versions.mk b/config/versions.mk
index c9274b8..d2fb3ba 100644
--- a/config/versions.mk
+++ b/config/versions.mk
@@ -4,6 +4,10 @@
 BLISS_VERSION_MINOR = 2
 # BLISS_VERSION_MAINTENANCE = Stable
 
+ifneq ($(SIGNING_KEYS),)
+    PRODUCT_DEFAULT_DEV_CERTIFICATE := $(SIGNING_KEYS)/releasekey
+endif
+
 # Bliss Android version
 BLISS_VERSION_STATIC = 11.0