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.

6LoWPAN: Modify the the way TCP headers are transferred -- yet again.  TCP packet reassembly now seems to work OK, but no there now does not seem to be a listener on the port.

6LoWPAN: Various fixes for a clean build if either TCP or UDP are disabled.  Given the current state of TCP, it is recommended that you disable TCP.

6LoWPAN TCP: Major re-architecting of TCP logic to properly handle TCP stuf like ACKs and TPC windowing which were not properly covered in the initial design.  Still does not work; hangs waiting of ACKs.

6LoWPAN: TCP logic was not obeying MTU packet size limitations.  Other TCP-specific issues also fixed.  There remains a major outstanding issue with ACK handling.

clicker2-stm32/mrg24j40-6lowpan: Add apps/examples/nettest.  Refresh all configurations that enable the nettest.