elemental: support building device images

[Itxaka]

This card is about to port required ARM scripts from https://github.com/rancher-sandbox/cOS-toolkit/tree/master/images as a standalone elemental command to build platform specific images.

Current build-disk is command is very focused on building vanilla/recovery images. Device images are full images, with extra scripts and so on, and they look closer to a full install than a vanilla/recovery image.

Investigate how to port those device scripts to the new elemental system