[oe] Major changes to ixp4xx and nslu2 machines, and slugos distro

[Rod Whitby]

The "ixp4xx" OE machine selection is now deprecated, and will tell you
to either use "ixp4xxbe" or "ixp4xxle" if you try to use it.

The "nslu2" OE machine selection is now deprecated, and will tell you to
either use "nslu2be" or "nslu2le" if you try to use it.

The "openslug" and "debianslug" OE distro selections are now deprecated
- you should replace them with "slugos".

SlugOS packages currently end in _arm.ipk or _armeb.ipk - this has now
changed to _armv5te.ipk and _armv5teb.ipk

The "ucslugc" distro is effectively unmaintained (and has been for a
long time).  If no-one steps up to maintain it, I'll be moving it to the
obsolete directory.

-- Rod Whitby
-- NSLU2-Linux Project Lead