- 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.

If useful for several boards, this should probably be provided directly by LiteX.

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.

eth-ip will also be used to configure Ethernet IP addresss.

targets/colorlight_5a_75x: update instructions and LiteEthPHYRGMII's tx_delay (required with LiteEth fixes).

This allows creating SoCs with CPU, SDRAM and Etherbone enabled all together.