Reorganization and cleanup of Web App Manager Yocto recipes and layers

From the previous comment: Rasberry Pi was also sucessfully tested. We are currently maintaining the recipes for guppy and master. This ticket can be set as done.

After the patch has been merged, cherry-picked to master: https://gerrit.automotivelinux.org/gerrit/#/c/19601/

Platform status:

• minnowboard: builds and works.

• R-Car gen.3: it builds, couldn't be tested due to the homescreen not launching (EDIT: doesn't seem to happen when WAM is not built, races between system services?)

• Raspberry Pi: untested.

While the gerrit review is in progress, I cherry-picked the change (https://gerrit.automotivelinux.org/gerrit/#/c/19271/) into a local gg/master checkout and started testing it. So far, it builds and runs on minnowboard. I'm pending to test other platforms but things are looking good.

Early push to https://gerrit.automotivelinux.org/gerrit/19271, to check CI pass. Although it builds on Raspberry Pi 3, I still have to check other platforms. Besides, some code was carried over from the original webOS recipe and I'm not convinced about it, like in chromium68.bb, line 28:

{{# we don't include SRCPV in PV, so we have to manually include SRCREVs in do_fetch vardeps
do_fetch[vardeps] += "SRCREV_v8"
SRCREV_FORMAT = "main_v8"}}

The team at Igalia has made great progress integrating Chromium 68, so I think it's not worth debugging the patch that merges Chromium 53 at: https://gerrit.automotivelinux.org/gerrit/#/c/18427/

The recipes are quite different, so I will create a new review and abandon that one.