scripts: add b4-wrapper for poky

scripts: add b4-wrapper for poky

poky is a combo-layer containing BitBake, OpenEmbedded-Core and Yocto
Documentation source code into one big repo. It is not uncommon to have
people develop patches for either of those projects from a poky git
repo.
However, it is unlikely those patches are to be sent to the poky mailing
list as very few files contained in the poky git repo actually are
poky-specific. So we need a way to identify to which mailing list a
patch is destined to be sent.
Additionally, because the source code in openembedded-core is
merged/imported at the root of the git repo of poky, its .b4-config
introduced in the previous commit will be used if not overridden (which
will be done in a separate commit specific to the poky git repo). We
need to provide a different .b4-config in poky.

Therefore, this wrapper is used to identify automatically which mailing
list a patch series needs to be sent to (via b4 prep --auto-to-cc) and
does some additional checks (via b4 prep --check) such as making sure a
patch doesn't modify two different projects at the same time or that
multiple projects are modified by different patches in the same patch
series.

This wrapper script is meant to be used by poky's .b4-config. Ideally
the b4 prep --check part could be offloaded to `patchtest` once it
supports running on source different from OE-Core.

Note that b4 makes sure that an address doesn't appear twice in the
recipient list. There's also no priority in the type of recipient list,
so if the address appears first in Cc and then in To, only the Cc will
be added. The opposite is true as well.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>