Documents steps to take if "Missing operating system" error message is printed after an iCubOS update

[traversaro]

While updating a COMEXPRESS CONGATEC-based system (iCubGenova09, see https://icub-tech-iit.github.io/documentation/icub_cpu_boards/icub_cpu_boards/) to iCubOS 8.0 based on Ubuntu 20.04, we experienced that the icub-head was not starting and instead an error:

Missing operating system

was just printed on the screen after the BIOS.

@davidelasagna found that the problem was that the non-UEFI variant of the disk partition was put before in the BIOS Boot Sequence with respect to the UEFI one, and this caused the problem. Re-ordering the boot sequence in the BIOS to have the UEFI disk before solved the problem.

While we solved the specific problem on iCubGenova09, if this problem happens to a iCub user outside IIT, it could be quite tricky to debug and solve for an iCub user. For this reason, it could make sense to document this somewhere, for example as part of some troubleshooting section of the documentation.