[Buildroot] howto incorporate buildroot into an autobuild

[John Stile <john@stilen.com>]

I am using buildroot for an embedded Arm project.

I was wondering how people using buildroot implement continuous
integration builds?

Do you start from a clean checkout, build the whole tool chain (which
takes a long time), and then build the project specific stuff?

I worry about not starting from a clean checkout, since some options may
change, and the tools won't be rebuilt.

But I also worry people will complain how long the build takes.

Is there a good strategy to balance these concerns?