dts: gen_defines.py: Save final devicetree to zephyr/zephyr.dts

Add a '--dts-out <file>' flag to gen_defines.py that saves the final
devicetree to <file>, in DTS format, using the new EDT.dts_source
attribute.

Handy to have available as a debugging aid. Unused otherwise.

Also write a dummy <BOARD>.dts_compiled file that tells people to look
at zephyr.dts, for people that might be used to that file. It was
removed in commit 8d8b0697 ("dts: Remove generation of
<BOARD>.dts_compiled").

Fixes: #22272

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>