configs: add nitrogen6sx_defconfig for Boundary Devices SoloX board

configs: add nitrogen6sx_defconfig for Boundary Devices SoloX board

This board is using the exact same kernel and u-boot revision as the
Nitrogen6x. They also share the same u-boot "autoboot" script.

The differences between the two configurations are:
- getty port is ttymxc0 instead of ttymxc1
- uImage load address is 0x80008000
- different device tree

http://boundarydevices.com/product/nit6_solox-imx6/

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>