bitbake: build.py: create symlink for run.do_xxx scripts

bitbake: build.py: create symlink for run.do_xxx scripts

The 'courtesy' symlink for log.do_xxx are quite useful when debugging, so
with this commit, we now get similar 'courtesy' symlink for run.do_xxx
scripts.

We only create symlink for tasks, not individual functions.

The symlink is create right before the actual runfile is created, indeed
we cannot create the symlink right after running the task since a failure
or execption can happen, in which case the symlink wouldn't be created,
and symlink are particularely useful when the task failed!

Another option would be create the symlink after the runfile is created,
and before the script is executed, but that means we need to duplicate the
code in case of Shell vs Python task.

(Bitbake rev: a672b39c5d529ba85d72eee8fef4c4273eaa5397)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>