Use a script to generate the branch/tag information inside poky.yaml.
If the branch isn't a known release branch, include git magic to find
the closest matching release branch we know about.
(From yocto-docs rev: 841e2df0e2e544b82fff9ddd0339a4e775148e3a)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>