Introduce Multi-DTB container support

[TravMurav]

lk2nd already implements a lot of checks to definitively detect which specific device model it runs on. This means that lk2nd could also use those checks to choose the DTB from a container instead of loading the single appended one. Implementing this would allow us to make more generic system images that could boot on all devices within a supported platform (platforms?).

The proposed approach is to use FDT based container for DTB images: This allows re-using libfdt to just select a node within the container by it's compatible. Then install this container as appended-dtb to re-use most of the boot path and just insert a check for unpacking the container.