patchtest: set default repo and testdir targets

patchtest: set default repo and testdir targets

Since patchtest is in oe-core, the Python os module's methods can be
used to retrieve the repository path and tests directory by default.
This reduces the number of mandatory arguments for invocation of
patchtest unless the user wants to use a custom test suite or test
patches against a different repo. The REPO and TESTDIR arguments are
likewise adjusted so that they are optional. Also, make it more obvious
what the --startdir flag is meant for on the command line by renaming it
to --testdir, and update the scripts/patchtest.README file to be
consistent with the new usage.

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