configs: am62a: use kernel fitImage when using secure bootflow

In order to maintain the chain of trust, each stage of the boot process
will first authenticate each binary it loads before continuing. To
extend this to the kernal and its dtbs we can package the kernal and
its dtbs into another fitImage for Uboot to authenticate and extend the
chain of trust all the way to the kernel.

When 'boot_fit' is set, indicating we're using the secure bootflow, look
for and authenticate the kernel's fitImage.

Signed-off-by: Judith Mendez <jm@ti.com>
Signed-off-by: Bryan Brattlof <bb@ti.com>