[SeaBIOS] SeaBIOS only tries to boot from first namespace of NVMe controller

[Thanos Makatos]

I discovered that SeaBIOS only tries to boot from the first NVMe namespace of an NVMe controller. Therefore, if the OS is installed in the 2nd namespace boot will fail. Is there a reason why all other namespaces aren't tried? I've looked at the code and IIUC each namespace is treated as a separate disk. Would we ever want to fix this? If not, maybe we should update the documentation accordingly?