nitrogen6x_defconfig: Add various USB serial devices

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
nitrogen6x_defconfig: use 3G/1G memory split

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
nitrogen6x_defconfig: Add support for Systemd

Add required and suggested elements to support systemd:
	http://cgit.freedesktop.org/systemd/systemd/tree/README

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