net/lwip : ND6 RA, add on-link prefix with valid length (<= 128)
net/lwip : ND6 RA, add on-link prefix with valid length (<= 128)

This commit is patch to add on-link prefix with valid length (<= 128)
 - as per RFC 4861,
   the prefix length is still valid for on-link determination
   when combined  with other flags in the prefix option.

Change-Id: I767489cb49240c79645de946275afaf5f46088d2
Signed-off-by: Jin-Seong Kim <c7ef485e0ddc445bcfb224daa3f265bce32dcc94@samsung.com>

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