shadow: update SRC_URI now Alioth is down

shadow: update SRC_URI now Alioth is down

Alioth is dead so we can't use it for SRC_URI anymore.  There is a shadow
repository on GitHub which is the new upstream, but for some reason it is
missing the 4.2.1 tag and tarball that we use.  Also 4.2.1 was never uploaded
into Debian itself, so we can't use their mirror network either.

For now point SRC_URI at the Yocto Project source mirror and set
UPSTREAM_CHECK_URI so that we get nagged to upgrade to 4.6.

(From OE-Core rev: b3e246fef166030f327b5a852718ea907ada1759)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>