Introduce a new approach to dtb-building:
Old way: `rsync` an overlay containing our dt source to the kernel,
and then build via buildroot's `_INTREE_DTS_NAME`. This does not work
in recent kernels (at least since 6.12), which requires all dtb's to
be explicitly listed in a `Makefile`.
New way: Create a separate package for every board, and build the
device trees via the kernel's standard interface for building external
modules. This has two main advantages:
1. We can easily interpose a patched version of an upstream device
tree. E.g., we can easily set the factory password.
2. It is much easier to get an overview of which boards are enabled,
rather than scrolling back and forth in the `_INTREE_DTS_NAME` list.
Enable all fully supported boards in the defconfigs.
Tobias Waldekranz