Squashed commit of the following:

commit f97da3a546f9d6e1b858cfc45538a06b62396034
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Sun Jul 30 19:09:46 2017 -0600

    Network: Condition out some types that depend on definitions that are only available with 6LoWPAN is enabled.

commit 312e9dc1195527100e12bf6be8b629f5b29be1f6
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Sun Jul 30 18:57:23 2017 -0600

    6LoWPAN: Fix case where source and destination IP address were backward.  Fix some compile issues when star topology support is enabled.

commit 3b0e71c5807194fbb006d0560bb2c98133055ba3
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Sun Jul 30 16:01:48 2017 -0600

    6LoWPAN PktRadio:  Finishes up all logic; Lots of misc changes from build testing.

commit cc118f8cb335e1c48354fd7112e20be57de50b68
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Sun Jul 30 12:41:18 2017 -0600

    6LoWPAN: Remove explicate type struct ieee802154_frame_meta_s from derive interface methods.  Replace with a opaque void * type s...