Fix Debian config for v5.11 builds

Clear out trusted key list as we don't have access to the ones expected
by the Ubuntu base config.

pkg/debian/kernel/ubuntu.config

@@ -4,6 +4,12 @@
 
 # CONFIG_DEBUG_INFO is not set
 
+##
+## Clear out trusted key list as we don't have them.
+##
+
+CONFIG_SYSTEM_TRUSTED_KEYS=""
+
 ##
 ## Additional options from upstream (not in PPA)
 ##