net/lwip: ND6 state management
net/lwip: ND6 state management

Change the state management for NS according to RFC 7.2.3.
If an entry does not already exist, the node SHOULD create
a new one and set its reachability state to STALE.
If an entry already exists, and the cached link-layer address
differs from the one in the received Source Link-Layer option,
the cached address should be replaced by the received address,
and the entry's reachability state MUST be set to STALE.

Change-Id: Ia11d1e7b13e17fab91a79465e4303a00d48df814
Signed-off-by: jisuu.kim <800dec47d2987cee7cc3d7aee71bc11b56f5042e@samsung.com>

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