docbook-xml-dtd4: use snapshot.debian.org for SRC_URI
Using ${DEBIAN_MIRROR} for SRC_URI doesn't work very well as that will
only contain releases that are currently in Debian. 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.
[YOCTO #10040]
Signed-off-by: Maxin B. John <maxin.john@intel.com> Signed-off-by: Armin Kuster <akuster808@gmail.com>
