That`s right - I am running bookworm off SD, since my bullseye system is still present on emmc.
The effect of omitting the "BUILD_ONLY" option in the compile instruction is simply that everything is being compiled from sources such that the bookworm image is compiled too. And 'current' was used, since bookworm was designed to operate with that version of linux.
A short test using 'hdparm -tT /dev/mmcblk1' #emmc indicates that emmc access speed with linux 6.1.45 is around 120MB/s (hs200), about half of what it used to be with linux 5.10.43 (220 MB/s = hs400).
I can live with that, but before I copy the bookworm system from sd to emmc, could you please confirm that linux 6.1.xx can safely be used to read and write emmc using the slower hs200 mode ?
P.S.: Regarding u-boot: before I became aware of your warning I already flashed the u-boot binaries to emmc using the blobs that came with the fresh bookworm image. emmc was still bootable and I did not oberve any issues. To be on the safe side, I reverted back to the original blobs present in my original bullseye image.