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>

ieee80215: starts adding support for beacon-enabled networking

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

6LoWPAN TCP: Fix more ACK handling, fix some callback flag settings.  Now the TCP test appears fully functional on 6LoWPAN.