nitrogen6x: config: allow use of dfu-util on SPI NOR

Enable the DFU driver and command.

Configure it for use on serial EEPROM.

Usage to program U-Boot is:
	~/$ sudo dfu-util -a u-boot -D u-boot.padded

Note the use of the padded U-Boot. You can make one
of these from u-boot.imx like so:
	~/$ dd if=u-boot.imx bs=1k seek=1 of=u-boot.padded.

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