[rt] Override SYSTEM_TRUSTED_KEYS to work from the source_rt directory

debian/changelog

@@ -6,6 +6,7 @@ linux (4.6-1~exp2) UNRELEASED; urgency=medium
   * [tilegx] linux-libc-dev: Install <arch/*> headers in arch-specific
     directory (Closes: #823632; thanks to Helmut Grohne)
   * [rt] Update to 4.6-rc7-rt1 and re-enable
+    - Override SYSTEM_TRUSTED_KEYS to work from the source_rt directory
   * Fold debian/config/README into debian/README.source
   * debian/README.source: Rewrite and expand explanation of kernel config file
     construction

debian/config/featureset-rt/config

@@ -4,3 +4,5 @@ CONFIG_SCHED_TRACER=y
 CONFIG_MISSED_TIMER_OFFSETS_HIST=y
 CONFIG_WAKEUP_LATENCY_HIST=y
 CONFIG_RCU_EXPERT=y
+#. Certificate paths are resolved relative to debian/build/source_rt
+CONFIG_SYSTEM_TRUSTED_KEYS="../../certs/benh@debian.org.cert.pem"