mcs814x: document how to include 3rd party binary packages

The dLAN USB Extender profile was referecing binary packages which are not to
be included in any of the source-only feeds. Instead remove these references
and document how to use the 3rd party feed.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 44071