Back port of upstream patch 2ff04a93:
Back port of upstream patch 2ff04a93:

    commit 2ff04a931ae85d0e1adc05b59afd95f9eaa4810d
    Author: David van Moolenbroek <david@minix3.org>
    Date:   Tue Dec 20 22:11:54 2016 +0000

	nd6: check link status before sending packets

	In the cases that nd6 checks whether the interface is up before
	sending a packet, also check whether the link is up.  Without this
	additional check, temporary link downtime could easily result in
	unnecessary false negatives for Duplicate Address Detection.

	In addition, use the netif abstraction macros to perform the checks.

openweave/openweave-core,openweave/openweave-core,openweave/openweave-core,openweave/openweave-core,openweave/openweave-core,openweave/openweave-core