Explicitly create /mnt when doing loop mounts for Linux ROMs

I found that, on the Glitch CAF kernel for flo, /mnt doesn't exist in the ramdisk. It causes any attempt to boot a Linux ROM (using the text file template from the wiki and an image instead of a subdirectory) to fail.

This creates it explicitly before it's needed. If it's already there, the mkdir call will fail, but there will be no adverse effects.