bitbake: Test premirrors and mirrors in checkstatus()
checkstatus() is used to ensure we can fetch a copy of each file, so it makes
sense to also test PREMIRRORS and MIRRORS in the method.
This patch adds calls to try_mirrors() to the Fetch.checkstatus() method and
changes the try_mirrors() method to take a check argument, which is False by
default. When check is True try_mirrors() will call a fetchers checkstatus()
with the replaced uri.
