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.

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.