dts: remove legacy macro support

The legacy macros were first deprecated in Zephyr v2.3. Now that
Zephyr v2.4 has been released, that makes two releases where these
macros have been deprecated, so it's OK to remove them.

This leaves support for legacy binding syntax in place. Removing that
is left to future work.

We need to update various pieces of documentation related to flash
partitions that never got updated when the new API was introduced.
Consolidate this information in the flash_map.h API reference page,
since that's really where users will run into it. This also gives us
the opportunity to improve this documentation.

Adjust a couple of kconfigfunctions.py and sanitycheck bits to use
non-legacy edtlib APIs.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>