UDP networking:  The TTL (time to live) was not being set in the IPv4 or IPv6 header unless the UDP socket was bound.

IP forwarding.  Adds a little more structure to handle passing packets received on one network device to another network device.  Logic is still incomplete.

samv71-xult: Fix SP0 vs. TWHS0 in mrf24j40-starhub/defconfig; fix copy-paste errors in SPI initialization logic.

samv71-xult:  Add support for the MRF24J40 radio and create a mrf24j40-starhub configuration.  A few fixes to IPv6 and 6LoWPAN were required to have 6LoWPAN and Ethernet coexisting.  Untested and expect some complexity in the bring-up.

Clang: Should there not be a prefix when using a cross compiler where clang refers to the host clang.

include/nuttx/net and net/: Move private prototype of netdev_foreach() from the public include/nuttx/net/net.h to the private net/netdev/ndetdev.h header file where it belongs.

6LoWPAN:  TCP send logic was returning a failure in one case when, in fact, the send was successful.

This change backs out the 'misconception' fix of 76e6dba2 and reimplements it in a way that actually seems to work.

6LowPan:  Change how the destination node address is handled in the start endpoint configuration.   When the star endpoint sent the IPv6 destination address, the HC06 compression logic elided the address -- meaning that it could be reconstructed by the recipient based on the receiver's assigned short address.  However, when intercepted by the hub, the uncompressed address does not know the short address of the recipient and instead uses the short address of the hub.  This means two things:  (1) it looks like  the hub address is the destination address, and (2) the uncompressed UDP packet has a bad checksum.  This change assures that the destination IPv6 address is not elided in the case of the star endpoint configuration.

Clicker2-STM32 + net/:  Add configurations to support the endpoint and hub roles in a star topology.  Fix various things that broke when IPv6 forwarding was enabled.

6LoWPAN:  Add configurable support for 6LoWPAN star topology.  With this change, the endpoints which are the 'points' of the star will forward all traffic to the coordinator.  The coordinator is assumed to be the 'hub' of the star.  This function also implements IPv6 forwarding.  That forwarding implementation is minimal, just enough to support the forwarding requirements of the star hub node.