scripts: dts: write new gen_defines.py

This is joint work with Kumar Gala (see signed-off-by).

This supports a new devicetree macro syntax coming. It's not really
worth mixing up the old and the new generation scripts into one file,
because:

- we aim to remove support for the old macros at some point, so it
  will be cleaner to start fresh with a new script based on the old one
  that only generates the new syntax

- it will avoid regressions to leave the existing code alone while
  we're moving users to the new names

Keep the existing script by moving it to gen_legacy_defines.py and
changing a few comments and strings around. It's responsible for
generating:

- devicetree.conf: only needed by deprecated kconfigfunctions
- devicetree_legacy_unfixed.h: "old" devicetree_unfixed.h macros

Put a new gen_defines.py in its place. It generates:

- zephyr.dts
- devicetree_unfixed.h in the new syntax

Include devicetree_legacy_unfixed.h from devicetree.h so no DT users
are affected by this change.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>