configs: engicam: Enable CONFIG_IMAGE_FORMAT_LEGACY

Enabling FIT along with Signature will make bootm to
not-understanding u-boot legacy image formats like uImage, etc.
So this patch enabling legacy image format for backward compatibility.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>