IP Forwarding: Add IPv4 packet forwarding logic.  Initial commit is an untested clone of the IPv6 forwarding logic with a few minor logic changes for IPv4.

IP forwarding: In check it the Ethernet MAC address is in the ARP/Neighbor table, add an additional check to skip in the case of CONFIG_NET_MULTILINK and the devices is not an Ethernet device.

configs/Board.mk:  Remove comment form end of line. In windows native build, it appears to be trying to make that an extra parameter to the AR command

I've found that the interrupts aren't enabled since nothing updates them after btn_poll() marks the file descriptor structure as being polling. I've managed to make it work with this change.

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.