tools: mkimage: add firmware-ivt image type for HAB verification

When we want to use Secure Boot with HAB from SPL over U-Boot.img,
we need to append the IVT to the image and leave space for the CSF.
Images generated as firmware_ivt can directly be signed using the
Freescale code signing tool. For creation of a CSF, mkimage outputs
the correct HAB Blocks for the image.
The changes to the usual firmware image class are quite small,
that is why I implemented that directly into the default_image.

Cc: sbabic@denx.de

v2-Changes: None

Signed-off-by: Sven Ebenfeld <sven.ebenfeld@gmail.com>
Reviewed-by: George McCollister <george.mccollister@gmail.com>
Tested-by: George McCollister <george.mccollister@gmail.com>