Add local mirror support to config.sh.

The repo tool supports downloading git repositories once to a local mirror
and then pulling from those repositories for separate project working areas.
This is done by passing either --mirror or --reference= to repo init. When
working with more than one project or branch this can greatly speed up the
sync process.

This patch adds support for local mirrors to config.sh.  To create a mirror:

    cd /some/path/b2g-mirror
    BRANCH=master ./config.sh mirror

To use the local mirror, you must set the B2G_LOCAL_MIRROR environment
variable before first executing ./config.sh.  For example:

    cd /some/other/path/B2G
    B2G_LOCAL_MIRROR=/some/path/b2g-mirror ./config.sh hamachi

This patch depends on the mirror.xml manifest being added to the b2g-manifest
repository.