Suggest caching Zephyr SDK for Travis speed / success improvement

[grgustaf]

This Travis build failed for me while trying to download the Zephyr SDK. I wonder if you're seeing that a lot? I used to have that problem in ZJS but I solved it with Travis caching in this pull request.

If it had been that simple here I would have tried it and submitted a PR right away, but it looks like you'd need to edit zephyr/Makefile.travis as well as .travis.yml and I don't have time at the moment to look into this, so I wanted to make a note of it.

This also has the benefit of skipping the download most of the time, speeding up the Travis build a lot. So basically, if you do it like I did, you'd cache some directory that holds the SDK and then check to see if it already contains the right version before you bother to download it with wget in the Makefile.

I'd also suggest parameterizing the Zephyr build version like we've done in ZJS to make it easier to update.

I intend to look at this someday myself but in case someone cares to beat me to it, here you go. :)

[LaszloLango]

A new error occurred on Zephyr job of Travis CI:

setuptools_scm.version.SetuptoolsOutdatedWarning: your setuptools is too old (<12)

[akosthekiss]

Is this still an issue? #2333 has solved the build error and the build time is under 5 mins for Zephyr.

[akosthekiss]

Closing as builds seem to be stable and build time is acceptable. Feel free though to submit a PR if you still think that caching should be applied to the Zephyr build (or to any/all of the other CI jobs).