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.

Fix warning implicit-fallthrough with arm-none-eabi-gcc 7

Approved-by: Gregory Nutt <gnutt@nuttx.org>

Fix ELF loader up_checkarch on ARM arch

Approved-by: Gregory Nutt <gnutt@nuttx.org>

net/: IPv6 packet input, Improve checks that the packet is destined for us.  There might be some odd things that can happen in certain forwarding scenarios.

Clicker2-STM32:  Add support for the Telnet client to the mrf24j40-6lowpan configuration.  Useless at the moment because the NuttX telnet daemon does not yet support IPv6.

stm32_hrtim

Approved-by: Gregory Nutt <gnutt@nuttx.org>