don't copy vmlinuz-dtb to /boot

When appending the dtb to the kernel, we would also copy the resulting file to /boot, however there is no use for this and it takes up precious space (the default size of /boot is 128M 256M). Stop copying it there.

Saves a whole 14MiB of space with the SDM845 kernel. I suggest we add a pre-upgrade script to the boot-deploy package which deletes this file to free up space.

EDIT by Oliver: default is 256M, I just had a very old pmbootstrap.cfg when I ran into this the other day