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.