Adding better handling for kernel cmdline arguments

[rdoxenham]

In this PR we introduce better handling of kernel cmdline arguments, specifically:

1. Strip out duplicates in both raw and iso boot, as to avoid relying on the order in which they're applied and ensuring that Kiwi-supplied kernel cmdline arguments aren't overriding user-supplied ones.
2. Remove the repliance on rd.kiwi.install.pass.bootparam which ended up being a bit of a hack just to get kernel cmdline arguments into the system post-kexec.
3. Insertion of desired kernel cmdline arguments into both iso and raw image booting so the resulting systems cleanly apply

Note that there's not yet a mechanism to remove kernel cmdline arguments, and the supplied code just ignores duplicated kernel cmdline arguments that do not have a value, as having these duplicated doesn't really matter.

Related: suse-edge/nm-configurator#122
Fixes: #604 #369