Thanks, @Dexogen, this was helpful for me. I found that the MODULES line wasn't needed, and specifically the linux-rk35xx-legacy kernel config file is used for these boards.
Here were the full steps I followed to successfully build an Armbian image with ceph RBD & cephfs support for a PVE arm64 installation. I built on an Ubuntu Jammy qemu VM running on an x86 Proxmox host, and ran as a normal user with sudo privileges as per the Armbian build docs. I needed to ensure the machine could reach external DNS (I think Cloudflare but I just allowed all outbound UDP/TCP 53) to build successfully - I only allow certain DNS servers in my network. You also need more than 10GB free space for the build to complete successfully. Ignore the warnings regarding shallow build.
git clone --depth=1 --branch=main https://github.com/armbian/build
cd build
sed -i 's/.*CONFIG_BLK_DEV_RBD.*/CONFIG_BLK_DEV_RBD=m/' config/kernel/linux-rk35xx-legacy.config
sed -i 's/.*CONFIG_CEPH_FS.*/CONFIG_CEPH_FS=m/' config/kernel/linux-rk35xx-legacy.config
./compile.sh build BOARD=orangepi5-plus BRANCH=legacy BUILD_DESKTOP=no BUILD_MINIMAL=no KERNEL_CONFIGURE=no RELEASE=bookworm
I also have some Rock 5b boards, you can simply replace 'orangepi5-plus' with 'rock-5b' in the above commands to build for that board.
It's also currently necessary to downgrade the bundled edk2 firmware for rk3588 to succesfully boot VMs. I used the following commands:
apt download pve-edk2-firmware=3.20220526-1 && dpkg -i pve-edk2-firmware_3.20220526-1_all.deb && rm pve-edk2-firmware*.deb
echo 'Package: pve-edk2-firmware
Pin: version 3.20220526-1
Pin-Priority: 999' > /etc/apt/preferences.d/pve-edk2-firmware
