net/lwip : neighbor cache should be updated when lladdr option exists on RA
net/lwip : neighbor cache should be updated when lladdr option exists on RA

This commit is including fixes as belows :
 - neighbor cache should be updated when lladdr option exists on RA
 - nd6_new_router only creates default router list without creating
   a neighbor cache entry

Change-Id: Ie138587501d1b348f92ec6729fd8bf351aba78ef
Signed-off-by: Jin-Seong Kim <c7ef485e0ddc445bcfb224daa3f265bce32dcc94@samsung.com>

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