cmd: bootz: fix device-tree overlap test

The address of the kernel image is stored in images->ep. zi_start is the
offset of execution entry in the image, which is usually 0 for ARM
zImage.

Fixes boot error when ftd is stored near RAM address 0x0:

ERROR: FDT image overlaps OS image (OS=0x0..0x5fd608)

Fixes: fbde7589 ("common: bootm: add checks to verify if ramdisk / fdtimage overlaps OS image")
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>