Add a linux-arm build for the OpenCV preset

[ThomasJClark]

This should make it possible to run JavaCV on non-android arm systems (probably including a Raspberry Pi, although I don't have one to try it on)

I've tested this on Fedora 23 using the ARM toolchains from lantw44/arm-linux-gnueabi-toolchain. Basically it just adds a linux-arm option to cppbuild.sh.

./cppbuild.sh -platform linux-arm install opencv
mvn install --projects .,opencv -Dplatform=linux-arm -Dplatform.compiler=arm-linux-gnueabi-g++

Closes #37

[saudet]

Oh, if a cross-compiler works, that would be neat. Have you actually tested on any ARM devices?

[ThomasJClark]

@saudet I'm in the process of trying to get this tested on a roboRIO. I have something compiled, but it'll be a little bit before I have physical access to the machine. I'll post an update then.

[saudet]

Cool. BTW, some people have been asking the same for FFmpeg. It doesn't look like it would be too hard to adapt it in the same way. Could you try it out and add it this PR if possible? :)

[JLLeitschuh]

👍 (same project)

[saudet]

BTW, even if you don't get FFmpeg done, let me know when you confirm this works on roboRIO or what not, and I'll merge this PR in! Thanks

[ThomasJClark]

It works on the roboRIo, just with the ARM toolchains replaced with a roboRIO-specific one.

I don't know if I'm gonna get around to doing ffmpeg. Sorry.

[saudet]

Ok, thanks. It would be nice to hear about someone trying this out on Raspberry Pi at least though...

[saudet]

Well, anyway, I'll merge this in, and we'll let someone else test it out and let us know if it works. Thanks for the contribution!