- main_ram mem_map is now directly used by add_sdram when origin is None.
- max_sdram_size/min_l2_data_width are no longer exposed as targets arguments this can
still be used enforced directly in the few cases it is useful.

targets: Switch to soc_core_args/soc_core_argdict (instead of soc_sdram that is now deprecated, but still supported for now).

Collect --with-ethernet/--with-etherbone, --with-spi-sdcard/--with-sdcard,
etc. into ArgumentParser.add_mutually_exclusive_group()s.  That way, we
get pretty --help output, and appropriate error messages if somebody
tries to ask for something that doesn't make sense.

Default to Chaser mode and similar user interface than GPIOOut.

As discussed recently on Discord.

- remove update in loading/flashing: we need to thinks how to integrate this.
- remove specific README: documentation is moved to the files, link to more complete project can
be added if maintained externally, as done for the iCEBreaker for example.
- revert default freq on ULX3S to 50MHz and instantiate a second PLL as done on the colorlight.

targets: default to trellis toolchain on all ECP5 targets (now able to build all supported targets).

We initially wanted to provide different level of support for the platforms/targets, mainly
to avoid too much maintenance and let each contributor update its contributed platforms and
targets, but it's easier to update all platforms/targets all-together when LiteX evolves or
changes (and that's what has been done on litex-boards since the creation of the repository).
So let just simplify things and avoid this differentiation.