bitbake/fetch: ensure the mirrored repository is updated as required

bitbake/fetch: ensure the mirrored repository is updated as required

If we fetch a tarball from a mirror it's entirely possible that the
mirror will have been from before the required tag/branch/etc was included
in the repository. To that end use forcefetch() as a way of testing whether
the repository is up to date and if not fetch updates.

Signed-off-by: Joshua Lock <josh@linux.intel.com>