make: pass mobile subserver prefix to docker when generating mobile RPC

mobile/gen_bindings.sh

@@ -57,7 +57,7 @@ done
 # If prefix=1 is specified, prefix the generated methods with subserver name.
 # This must be enabled to support subservers with name conflicts.
 use_prefix="0"
-if [ "$prefix" = "1" ]
+if [ "$SUBSERVER_PREFIX" = "1" ]
 then
     echo "Prefixing methods with subserver name"
     use_prefix="1"