RiOTboard and MarSBoard: add new boards support

RiOTboard is produced by Embest/Element 14 and is based on i.MX6 Solo
The following features are tested :
- UART2 (console)
- eMMC
- SDCard
- uSDCard
- Ethernet
- USB Host (through 4 ports hub)
- HDMI output
- I2C 1/2/3
- LVDS TFT with LCD8000-97C from Embest/Element 14

Boot on eMMC and through USB loader are tested.

For more informations on this board : http://www.riotboard.org/

MarSBoard is produced by Embest/Element 14 and is based on i.MX6 Dual
The following features are tested :
- UART2 (console)
- eMMC
- uSDCard
- Ethernet
- USB Host (through 2 ports hub)
- HDMI output
- I2C 1/2
- SPI NOR Flash
- LVDS TFT with LCD8000-97C from Embest/Element 14

Boot on SPI NOR and through USB loader are tested.

For more informations on this board :
http://www.embest-tech.com/shop/star/marsboard.html



Both boards are supported by the same code base as they are based on a
common trunk of schematics.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Acked-by: Stefano Babic <sbabic@denx.de>