opkg: add opkg-nogpg-nocurl-slugos recipe, update slugos.conf to use it.

opkg: add opkg-nogpg-nocurl-slugos recipe, update slugos.conf to use it.
This is a temporary measure made necessary by a major set of changes to
the entire set of opkg recipes that not only rendered older versions of
opkg sources unbuildable, it also removed a patch that has not yet been
accepted by opkg upstream.  Until these issues can be resolved and
adequate testing on small-memory devices performed, this copy of the
original recipe is a necessary evil that just has to be tolerated.