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.

The SoC reset added recently creates a path between sys_clk and pll.clkin
clock domains that is reported by the tools but that can be safely ignored.

Tested with:
./pano_logic_g2.py --uart-name=jtag_uart --build --load
./litex_jtag_uart.py --config=openocd_xc6_ft232.cfg
lxterm /dev/pts/X

Default to Chaser mode and similar user interface than GPIOOut.