Portable BIOS config + check and log if a valid BIOS is not found (#155)

* - BIOS in game config now is stored by name instead of full path, making it more portable

* - added check if bios folder is empty, logging in that case an explicit message
2 jobs for main in 2 minutes and 8 seconds (queued for 4 seconds)
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passed libretro-build-linux-x64 #867825


passed libretro-build-windows-x64 #867824