Add support for Linux and MacOSX

[smurfy]

FlashTool currently compiles on these Platforms, but avrdude.exe is hardcoded.
We need a switch to call the correct avrdude executable by platform.

[SirAlex]

Is it possible to not use avrdude? (Of course it possible, I'm don't know it's hard to implement it or no)

[smurfy]

Is it possible to not use avrdude?

yes it would be possible and that was one of the first things i looked at while thinking about the flashtool.

mission planner uses a c# port of the avrdude source to do so (https://github.com/diydrones/MissionPlanner/tree/master/Arduino).

I also took a look at the code from avrdude directly and i think it would be possible but i'm afraid borking my arduino / crius in the process :) don't know if i can make it unflashable somehow.

for example the apm_planner (extended version of qground control) uses avrdude again. i will raise an issue there and ask if they plan to use it nativ. flashtool and apm_planner are both c++ and qt so it would be easy to share it between the projects.