embestmx6boards: Change default bootenv configuration

Change CONFIG_MMCROOT and CONFIG_SYS_MMC_ENV_DEV in
order to have a smooth boot process with sdcard image
built from meta-fsl-arm-extra.

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