For anybody which might be interested I explain in short (for advanced users and untested) how to start to to manually install a minimal distribution of openmamba:

1. boot any Linux OS with networking
2. use i.e. curl or wget to download the openmamba rootfs-base archive from the download page
3. initialize the destination storage by creating the partition table (GPT or MSDOS) and a FAT32 EFI boot partition (2-300 MBytes) and an ext4 partition
4. format the partitions respectively with the mkfs.vfat and mkfs.ext4
5. mount on some mount point the O.S. partition (ext4) and bind mount the boot partition at /boot, aso bind mount /dev,/sys,/proc,/run from the running O.S.
6. run chroot to get an openmamba chroot shell in the O.S. partition
7. install the kernel (dnf install kernel-mamba-x86_64)
8. install grub packages (dnf install grub grub-efi-x86_64)
9. set a login password for the root user with passwd command
10. exit from chroot, unmount and try rebooting
11. you expected that probably something might not work, this can be discussed and fixed