Fixes for wltests on P3

Created by: qperret

So it turns out repo sync -l can rebase stuff behind your back ... I'm a bit tired of playing around with this so I took the decision to stop messing around with repo sync and to just use proper repo branches (which is what I should have done from the start). That makes it a bit harder to test different toolchains and things like that, but that actually works properly for what we really want to do, that is, testing kernel changes.

I understand this whole thing is becoming more confusing than necessary (since I change my mind every week or so), so this PR also creates some basic wltest doc to explain how to use it for 'repo' targets.

For the future implementation of wltest we should drop the support for !repo targets I think -- Google now ship build scripts for all platforms we have in wltest (https://source.android.com/setup/build/building-kernels#downloading) but I think that's for later.