- 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.
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.