sandbox: rename overlay sources to .dtso

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>
Reviewed-by: Simon Glass <sjg@chromium.org>