Igor Opaniuk authored Sep 14, 2020 and Stefano Babic committed Sep 17, 2020

Enable and set preboot var with fdtfile evaluation.
preboot 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.

Fixes: 304042c1

("colibri_vf: set fdtfile for distroboot")
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>