verdin-imx8mm: use preboot for fdtfile evaluation

Enable and set preboot var with fdtfile evaluation.
It will be checked and run immediately before starting the
CONFIG_BOOTDELAY countdown and/or running the auto-boot command resp.
entering interactive mode.

This provides possibility to use different boot cmds in interactive mode
without manual setting fdtfile value, as it it's already evaluated
before entering interactive mode.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>