nitrogen6x: config: add 'rundfu' for reference

The dfu command line is kinda hairy, at least when used
with serial flash. The third parameter contains a
colon-separated string with a bunch of parts:

	U-Boot > dfu 0 sf 0:0:25000000:0

In English, this is
	spi bus 0
	chip-select 0
	25 Mhz speed
	SPI mode 0

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>