linux-lts-utopic: Added CONFIG_DELL_RBTN=m

The package was failing to build due to that configuration.

I'm not sure why we didn't inherit this configuration from ubuntu kernel config, but adding it to config.common.trisquel does the work.

helpers/make-linux-lts-utopic

 #!/bin/sh
 #
 #    Copyright (C) 2008-2015  Ruben Rodriguez <ruben@trisquel.info>
+#    Copyright (C) 2015       Santi Rodriguez <santi@trisquel.info>
 #
 #    This program is free software; you can redistribute it and/or modify
 #    it under the terms of the GNU General Public License as published by
@@ -59,6 +60,12 @@ CONFIG_IOSCHED_BFQ=y
 CONFIG_CGROUP_BFQIO=y
 # CONFIG_DEFAULT_BFQ is not set
 EOF
+
+#Enable Dell "airplane mode" button
+cat << EOF >> debian.master/config/config.common.trisquel
+CONFIG_DELL_RBTN=m
+EOF
+
 cp debian.master/config/config.common.trisquel debian.utopic/config/config.common.trisquel
 
 cat << EOF >> debian.master/config/i386/config.flavour.lowlatency