Updates the routes on an FTD child based on a received `RouteTlv` from the parent. MUST be called when device is an FTD child and @p aRouteTlv is received from its current parent.
Fills a Route TLV. When @p aNeighbor is not `nullptr`, we limit the number of router entries to `kMaxRoutersInRouteTlvForLinkAccept` when populating `aRouteTlv`, so that the TLV can be appended in a Link Accept message. In this case, we ensure to include router entries associated with @p aNeighbor, leader, and this device itself.