Distinguish more clearly between source files meant for producing .dtb
from those meant for producing .dtbo. No functional change, as we
currently have rules for producing a foo.dtbo from either foo.dts or
foo.dtso.
Note that in the linux tree, all device tree overlay sources have been
renamed to .dtso, and the .dts->.dtbo rule is gone since v6.5 (commit
81d362732bac). So this is also a step towards staying closer to linux
with respect to both Kbuild and device tree sources.
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
