Align the RIOT target with the clang build of RIOT OS

[akosthekiss]

This means cross building with clang and using short enums. Also
bump RIOT OS version to latest 2018.07.

Note: On Travis CI, clang-3.9 is used for testing. That version is
widely available, from Ubuntu 14.04 to 18.04.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

[akosthekiss]

Related to and pending on #2473 : this PR will have to be rebased onto master when that one lands (right now, I've cherry-picked the fixes from that PR to make things build).

cc @miri64 : this PR may help the "developers wont have easy ways to detect the need for such casts" issue on the long run.

And all this also means that we may finally have a clang build test on the CI.

[miri64]

Cool! However, the current configuration makes it seem like RIOT is exclusively built with clang. I want to point out that this isn't the case. We are just currently hardening our clang support ;-).

[akosthekiss]

Thanks for the feedback. Duly noted. I'll put some extra words into the commit message to make that clear.

[akosthekiss]

Now that #2473 has landed, the PR is rebased onto latest master. It's ready for review.

[LaszloLango]

LGTM

[robertsipka]

LGTM