Single-disk M3U handling appears broken

[rhester72]

Describe the bug
If only a single entry appears in a M3U file, when saving the same config, it saves as [core]@[game].[image ext].cfg, not [core]@[game].m3u.cfg, but is properly read as m3u.cfg on launch.

To Reproduce
Steps to reproduce the behavior:

1. Create a M3U with one entry (e.g. Populous.m3u contains Populous.adf)
2. Start the M3U
3. Make any configuration changes you like, then save the game config (which will be written as /media/[drive]/config/settings/game/[display]/commodore_ami@Populous.adf.cfg incorrectly)
4. Restart the game
5. Note your changes appear to be lost
6. Rename /media/[drive]/config/settings/game/[display]/commodore_ami@Populous.adf.cfg to Populous.m3u.cfg
7. Restart the game again and your changes appear

NOTE: This probably affects all single-line M3U entries, not just the Amiga core, but I have not tested for this

Expected behavior
Single-line M3U configuration settings should be saved as *.m3u.cfg in the settings directory

Setup:

• Device: Raspberry Pi 500
• OS version: v1.7.0
• Storage: USB
• Power adapter: Raspberry Pi official