UBUNTU: [Packaging] prevent duplicated entries in modules.ignore

UBUNTU: [Packaging] prevent duplicated entries in modules.ignore

If we always append the list of disabled modules to modules.ignore we
may end up having lots of duplicate entries during the kernel cranking
process.

Prevent this by checking which modules are already present in
modules.ignore before appending them.

NOTE: it would be better to generate modules.ignore using some
information from the builder itself, rather than maintaining all the
*.ignore files. But for now, as a first step, let's prevent the
duplicate entries at least.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>