custom boot sound support [Ryuinferno]

- Place custom boot sound PowerOn.ogg in /system/media
- To mute, create an empty file named mute in /system/media
- To unmute, create an empty file named unmute in /system/media
- To restore original boot sound, create an empty file named ori_sound in /system/media
- Reboot device after each process
- modified by UpInTheAir@XDA for P6810 ICS