Handle kernel arguments better, especially when duplicated

[rdoxenham]

Earlier this week, an issue was raised in nmc (suse-edge/nm-configurator#122) related to the way that EIB is handling and configuring kernel arguments, and how in this specific case, net.ifnames=1 wasn't being honoured due to the way that SL Micro 6.0 kexec's into the new disk image post image dump, and uses a pre-configured kernel cmdline set at Kiwi image build time.

In short, we end up in a situation where a user can specify:

operatingSystem:
  kernelArgs:
    - net.ifnames=1

Yet the resulting kernel cmdline after kexec adds in the Kiwi-set ones, which by default includes net.ifnames=0, and thus ignores the user-provided argument. Earlier in the year, @e-minguez raised #369 which should also be considered where we simply rely on the ordering of the kernel cmdline arguments, but we should handle this better.