Create "build" directories for building and then add to .gitignore

[tomlogic]

I'm having to sort through lots of files in my git client, and it's hard to tell what needs to be added to the repository and what is just a configure/build artifact. It would be great to have configuration and building take place in separate directories. This would be for both building openrabbitfu and coldload.bin.

[spth]

WhileI see the point I don't dare to make real changes to the build system; I don't know autotools well, as you can see from #10 and #19.

[spth]

As of today (3d55f38), the out-of-tree build should be working the normal autotools way: You create your build directory, call configure from there, and then OpenRabbit will be build there. Please check if this is fine for you.