Restore compatibility of image type IDs

Commit a2b96ece breaks the enumeration
of the Image Types. New image types can be appended, but they cannot be
inserted in the list else backward compatibility is broken.

This restores the images types as before 2018.11 and move i.MX8 related
images at the end.

Signed-off-by: Robert Berger <robert.berger@ReliableEmbeddedSystems.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Tom Rini <trini@konsulko.com>