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.

Build triggered.

Source package build finished. Result OK Build results available at: http://jenkins.trisquel.info:8085/job/request-merge-source-builder/213/

Binary packages built and avaliable

mentioned in commit 687a4db7

Status changed to merged