Merge patch series "finish using .dtso for overlay source files"

Rasmus Villemoes <rasmus.villemoes@prevas.dk> says:

This is a followup to the patches that landed in 2024.01 and nearly
made sure that source files for producing .dtbo files use the .dtso
extension. In the same release, a few new .dts files snuck in, and
there was also some test code involving .dtbo -> .dtbo.S -> .dtbo.o I
didn't really know how to handle at the time. This should finish the
job, bring us in sync with linux (at least in this respect), and drop
the .dts -> .dtbo build rule.

diff --cc scripts/Makefile.lib
Simple merge