move the OS boot entry to the end instead of deleting it completely

Some firmware will just ignore BootNext and delete the entry if it's not
present in the order. Moving it to the end should serve the same
purpose.

Bug: 1031876
Change-Id: I15314eba588662e3b8a1674e92bbeb0d8def9120
Enable iso import for RHEL6.

This is due to the realisation that we can never rely on anaconda
to provide a consistent interface for isntalling isos, so we may
as well allow any distro to try its luck.

Also a couple of minor tweaks suggested from the original patch.

Change-Id: I54ae32c67b3afe0cefe68cac55e9310f77bd42b9