ARM: imx: cm_fx6: env: support distro boot command

The current default environment of the cm_fx6 is not suitable for
booting modern distributions.

Instead of extending the custom environment, let's use the distro
boot command, which has been developed for precisely this use case.

If the distro boot command fails, fall back to the old behavior
(except for USB drives where the old behaviour is completely covered
by the distro boot command). That way it is still possible to create
"rescue SD cards" for old installations (e.g. if one messes up the
on-flash environment).

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>