layerscape: harmonize device strings

layerscape: harmonize device strings

OpenWrt lately has harmonized device (definition) names to the
pattern vendor_model to improve overall consistency, also with
other values like the DTS compatible.

This patch applies that scheme to the layerscape target.

Since this (intentionally) creates a bigger overlap between DTS names,
compatible, and device definition name, it also moves DEVICE_DTS and
SUPPORTED_DEVICES definitions to the Device/Default blocks.

Apart from that, it also modifies several packages to use consistent
naming in order to keep the $(1) file references working.

While at it, remove one layer of complexity for the setup in
tfa-layerscape package.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>