[ip6] fix default source address selection (#4082)
[ip6] fix default source address selection (#4082)

Thread defines a mesh-local scope for IPv6 unicast addresses. However,
the mesh-local scope can only be inferred by inclusion of the
mesh-local prefix.

This commit updates the IPv6 source address selection to infer the
destination address scope based on a prefix match with currently
assigned addresses.
abtink/openthread,openthread/openthread,srickardti/openthread,turon/openthread,turon/openthread,bukepo/openthread,bukepo/openthread,jwhui/openthread,chshu/openthread,lanyuwen/openthread,bukepo/openthread,bukepo/openthread,openthread/openthread,librasungirl/openthread,librasungirl/openthread,jwhui/openthread,chshu/openthread,openthread/openthread,lanyuwen/openthread,srickardti/openthread,abtink/openthread,lanyuwen/openthread,chshu/openthread,turon/openthread,jwhui/openthread,turon/openthread,srickardti/openthread,chshu/openthread,srickardti/openthread,librasungirl/openthread,abtink/openthread,lanyuwen/openthread,lanyuwen/openthread,openthread/openthread,chshu/openthread,abtink/openthread,jwhui/openthread,chshu/openthread,librasungirl/openthread,lanyuwen/openthread