implement bootloader for selfupdate via audio

some project out there support this already.
at least i was told so. needs investigation later.



http://www.hobby-roboter.de/forum/viewtopic.php?f=4&t=127
http://wiki.8bitmixtape.cc/#/4_4.1-Hex2Wav
http://wiki.8bitmixtape.cc/#/4_4-TinyAudioBoot
https://github.com/ChrisMicro/TinyAudioBoot