Adding support for live-images configs and arbitrary configuration directories in lb_config --config option.

mdadm creates a /etc/mdadm/mdadm.conf configuration file
on package installation which contains array information
from the build system.

Dropping ubuntu guards on firmware selection code, we're not enabling firmware inclusion in ubuntu mode anyway.

Also, we're not keeping this as ubuntu should just update their archive
structure for content files to match debian. Not worth keeping temporary
extra turns just for ubuntu.

Correcting variable typo when setting distribution specific defaults of the backports mirror for the binary image.

Updating derivatives archive-key signature validiation to look by default at both debian and debian-maintainers keyrings.