This way we can group together the builds that are fast so we pay the
"setup price" just once. Let the long PX4 build on its own VM because
it's the one that takes most of the time building NuttX.
By grouping the projects by target it's also easier to split the build
in more VMs if we want to speed up the build.
Travis build is timing out because it's taking too much time. Let's
split the build so 1) they can run concurrently and 2) we don't timeout.
We still lose some time due to multiple checkouts, but at least we don't
fail the build due to timeout.
