Revert "tree: imx: remove old fit generator script"

This reverts commit d9a6f0ee.

For right now, it's too close to the release to merge the series that
allows for binman to be used to generate the final images, and also not
break CI, and then also merge all of the series that convert currently
broken platforms to use binman instead.  So, bring back this script now
and remove it again for real after the release.

Suggested-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Tom Rini <trini@konsulko.com>