Detect ISOs on all MultiOS-USB partitions

[ebbez]

This PR adds detection of ISOs in /ISOs on any partition of the MultiOS-USB device. Useful if the user wants a separate partition to boot large ISO files like Ubuntu or Windows from a partition with a different filesystem that is supported by the ISO environments.

I have tested the following to check if it works as expected:

• Boot Fedora from second partition (main MultiOS-USB partition) still works
• Boot from Ubuntu and Windows from a third NTFS partition has been succesful
• config_priv Ubuntu example boot from third partition also works
• Also tested a drive with 4 partitions and filesystems, and put a different ISO on each of them (EFI, FAT32=archlinux, NTFS=Ubuntu, ext4=Fedora) and they were all detected and booted correctly.

In many of the config files a parameter has been added to the submenus, to set iso_path to a relative path (/ISOs/distro.iso) instead of an absolute path (like (hd1,gpt2)/ISOs/distro.iso) that only GRUB can parse.

Potential downsides:

• (Performance). Since every config file has a search pattern with ($dev,*) it could be that with more partitions the search/detection of ISOs will take longer. Perhaps a better solution would be to run a for loop for all devices from the main grub cfg.

[Mexit]

Potential downsides:

* (Performance). Since every config file has a search pattern with `($dev,*)` it could be that with more partitions the search/detection of ISOs will take longer. Perhaps a better solution would be to run a for loop for all devices from the main grub cfg.

This is the biggest problem.

In general, the way ISO files are searched for is very slow and should be changed. I didn’t expect the project to grow this much. It was meant to be a temporary solution.
Ideally, all ISO files should be discovered first, and then only the specific files containing configuration should be loaded. That would be the fastest approach. I worked on this some time ago, but due to a lack of time, it’s not ready yet.