Makefile: Build firmware-ivt image type for HAB verification also for mx7

Create u-boot-ivt.img and u-boot-ivt.img.log when building U-Boot
with SPL and Secure Boot enabled for imx7 (like it is done for imx6).

See commit d21bd69b for more info.

Signed-off-by: Eran Matityahu <eran.m@variscite.com>