Add the riscv64 page to the cluster management guide (UEFI + grub2 boot
path, discovery through mknb's grub2 network configurations, stateful and
stateless provisioning, the dependency picture for a management node on
riscv64, limitations), list riscv64 in the node object attributes and in
the support matrices, add the architecture to the cross-build page for
stateless images, extend the grub2 install guide (and fix its swapped
x86_64/aarch64 file names), the uninstall package lists, the DHCP backend
validation matrix and the mknb/genimage man pages, and carry the riscv64
schema values into the generated nodetype, osimage, noderes, node and
group references.
Also update the stateless image creation file to include
the NOTE for using a local file. I hit problems with genimage
when it's writing to the mounted NFS /install directory