Add /etc/platform-build-info in images, SDK and deploy dir

Close for GG RC2

Both could make sense. But after some more thinking, we'll hit some issues when binary feeds for SDK will be up: SDK will be upgraded while the top level information will remain the same.

So ATM, I'll drop the information from the SDK and keep it only in the image and in the deploy folder. That will also make the recipe simpler.

If we see that information is needed from native tools, it will be time to add a native package (later).

OK, so the intention is to have, once the SDK is installed, a top-level file with contents, or is the intention that without installing the SDK the user has this file available?

Follow-up of Thomas's comment on gerrit #16723:

 The file platform-build-info was  intentionally  added in SDK root dir, to be able to extract these information easily.

That said, nothing prevents from distributing the same file inside SDK sysroots (in sysroots/*/etc/), so that at least native tools can benefit from these data. If not in multiple sysroots, that can be in the native sysroot only.

Could we do that in a second step, if we have a use case for it ?

Question: is it better to have git repos revisions or details layer by layer ? Current implementation simply parses all git repos in the top folder relative to aglsetup (=meta-agl/..)