Fixed UEFI config (bnc#865871)

This is a decent backport of the fix against next crowbar version. It
already have been better integrated with
https://github.com/crowbar/barclamp-deployer/pull/216 and
https://github.com/crowbar/barclamp-provisioner/pull/323.

This fixes https://bugzilla.novell.com/show_bug.cgi?id=865871 for roxy
release.
@vuntz This bit destroyed the UEFI before. ```dup``` or ```freeze``` on a nil value results in an exception :)