When playing with unetbootin or some other software for converting a ISO image to a USB stick, you can get, trying to boot from USB, a message like this:
ERROR: No configuration file found. No default or UI configuration directive found! Boot:_
You can fix this formatting the USB device before using it with FAT (instead FAT32).
YMMV, for example to fix the same error from the BAT installer of OpenELEC, you have to change this line
ECHO. | >NUL format %DRIVE% /V:OPENELEC /Q /FS:FAT32 /X
ECHO. | >NUL format %DRIVE% /V:OPENELEC /Q /FS:FAT /X