Selecting 4k page on Pi 5 prevents the kernel from being updated

[pelwell]

rpi-update contains logic to detect that a custom kernel is being used, disabling the updating of the kernel and modules if found because the dtbs may not be compatible. Unfortunately, using kernel=kernel8.img to force a Pi 5 to use the Pi 4 kernel (in order to use 4kB pages for compatibility) causes a false positive.

I think the logic needs to be expanded to ignore cases where it's a standard kernel name, or perhaps just kernel8 on a Pi 5.