imx6dl-riotboard: Add support for RIoT board

Add support for RIoT board (http://www.riotboard.org/) using by default
linux-fslc and u-boot-fslc.

The machine name follows the linux (kernel.org) naming convention.

The generated uboot is configured by default for usage on the external SDCard
and not for the internal eMMC.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>