classes/seppuku.bbclass: Switch to HTTP post, attach the build logs instead of using...

classes/seppuku.bbclass: Switch to HTTP post, attach the build logs instead of using comments
    The limit of comments is too low. Use attachments to work around this
    issue. Also start using HTTP post to avoid really long URLs. They lead
    to a pipe error.
    To post we use a MultiPartPoster which is licensed LGPL and can be freely
    downloaded. You need to put it into the PYTHONPATH
    Add a new variable for creating attachments and document it.