patchtest: add scripts to oe-core

patchtest: add scripts to oe-core

Add the following from the patchtest repo:

- patchtest: core patch testing tool
- patchtest-get-branch: determine the target branch of a patch
- patchtest-get-series: pull patch series from Patchwork
- patchtest-send-results: send test results to selected mailing list
- patchtest-setup-sharedir: create sharedir for use with patchtest guest
  mode
- patchtest.README: instructions for using patchtest based on the README
  in the original repository

Note that the patchtest script was modified slightly from the repo
version to retain compatibility with the oe-core changes.
patchtest-send-results and patchtest-setup-sharedir are also primarily
intended for automated testing in guest mode, but are added for
consistency.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>