Make flightmode, switch and system file names case insensitive

Consider this enhancement that makes openTX sound files case insensitive. 

The FAT file-system itself is case insensitive, but it nonetheless preserves filename case. The openTX was case sensitive, which resulted in slight problem, for example:
 * flight mode name: Speed
 * filename for that flight mode: speed-ON.wav

OpenTX searched for "Speed-on.wav" an was unable to find it, but it was in fact available under different case. This pull corrects that.