added links for the PREMIRRORS and MIRRORS variables used in
the description.
(From yocto-docs rev: 1ec0f9cbeebeaeaadee8cc8b3631e6194c013a2f)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>