Evaluate using buildroot DTS build/install of westermo device tree files

[troglobit]

Note: make sure base directory is copied, or at least the product-model-number prefix is retained so developer builds (its/itb files) work as intended.

[wkz]

Long-term I think we can:

• Drop the per-product kernel support in the current layout (which we've never used anyway)
• Move to a flat layout under /boot/westermo/

So we would transform /boot/1234-prod-desc/device-tree.dtb -> /boot/westermo/1234-prod-desc.dtb for example, and always combine that with /boot/*Image.

Barebox must then be updated to have two patterns in which to search for the DTB.