apalis/colibri_imx6: move setting bootcmd to defconfig

Move setting the default boot command to the
apalis/colibri_imx6_defconfig. It allows replacing the command
without code modification.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>