[Merge] ~itrue/livecd-rootfs:add-nemos-support into livecd-rootfs:ubuntu/master

[Isaac True]

@lool Thanks for the comments. I've moved the path to the NemOS descriptions from the build file to the config file where all of the other NemOS-specific configurations are.

Regarding arm64+aarch64, this is more of a side-effect of the fact that arm64 is called aarch64 in Kiwi-land, thanks to its SuSE heritage. Kiwi expects aarch64, whereas livecd-rootfs expects arm64, so I just call the default SUBPROJECT for arm64 "aarch64". 

I can talk with the customer about maybe changing the name from aarch64 to arm64 inside the Kiwi descriptions.
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https://code.launchpad.net/~itrue/livecd-rootfs/+git/livecd-rootfs/+merge/440173
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