arm: use _start instead of __image_copy_start

arm: use _start instead of __image_copy_start

TX53 creates an FCB at the start of the ELF image. This offsets _start
from __image_copy_start leading to relocation errors.

Temporary fix this by using _start instead of __image_copy_start.
A better fix would be to implement a 'romupdate' command, that takes
care of creating the FCB at runtime.