Setup CI for both native and cross compilation

[matthew-reynolds]

🚀 Feature Request

Currently, Travis only performs native builds. Not sure if it's possible, but it would be great if we could also test our cross builds. Might be able to use something like Qemu, but we need to be careful about not exceeding Travis's job time limit. I don't know if this is possible, but perhaps we could have a job for cross-compiling ROS and have Travis aggressively cache the result, and then run our cross-compile job against the cached result so that we don't need to rebuild ROS every time. Again, no idea if that's possible with Travis.

We should also download and build with all the supported 3rd party plugins enabled.

[matthew-reynolds]

Instead of Qemu, this would probably be better with a custom Docker image equivalent to NI Linux-RT or the FRC RoboRIO image