"I modified MAAS to only provide GRUB and skip the Shim" => that will not work in secureboot without canonical CA certificates provisioned. If only MS certificate is provisioned (default for most x86 hw) then one must use shim too.
"I modified MAAS to only provide GRUB and skip the Shim" => that will not work in secureboot without canonical CA certificates provisioned. If only MS certificate is provisioned (default for most x86 hw) then one must use shim too.