Using ${DEBIAN_MIRROR} for SRC_URI doesn't work very well as that will only
contain releases that are currently in Debian, so currently doesn't contain
1.18.7 as unstable has moved on to 1.18.9.
So, move all of SRC_URI to the .bb so it can use snapshot.debian.org instead,
and set UPSTREAM_CHECK_URI to ${DEBIAN_MIRROR} so upstream release checking
continues to work.
Signed-off-by: Ross Burton <ross.burton@intel.com>
