Hmm, i dug a little into the u-boot sources and found, that a boot partition apparently needs to be kept on the sd-card for now even if a nvme device is present.
The reason is, that the mtd device (cat /proc/mtd) is too small to contain the linux kernel and the current u-boot cannot read the kernel from nvme. While u-boot does have a nvme subsystem, it may lack a suitable driver in the moment. Please correct me, if i’m wrong here.
This does not preclude to use the sd slot normally. One can savely unmount /boot if the sd slot is really needed, since the boot partition on the sd is only needed during the u-boot stage.