Wltest on P3

Created by: qperret

Marking this as WiP since it needs further testing to make sure it is robust enough.

This is basically the least horrible way I could find to integrate the Google build system thing into the wltest infrastructure. The basic idea is to set WLTEST_REPO_TARGET=y in the platform's definitions file, and then check that in test_series to know if we're dealing with a 'normal' kernel source tree managed with git, or with a 'repo' tree using manifests and all.

The nice thing is, the API stays the same. You just need to make --kernel_src point at your repo, give a series of sha1s for the repo manifest, and things should 'just work'.

The part I don't like is that we're making test_series play with manifest sha1s, not kernel sha1s. And so that makes it a pain to check if we've flashed the right kernel on the target since obviously the kernel and manifest sha1s are different. So I basically disabled the checks for now.

The alternative would be make --kernel_src point at the actual kernel source tree, and to have an additional --repo_path parameter, or something like that. But that makes the command line even worst than it already is so I'm not sure if it's worth it.

The instructions on how to use that thing are in one of the commit messages. We should probably try and find a better place to store them, I guess.

Feedback welcome !