Add retries to commands that download from external servers.

This helps reduce the chance of a build failure due to transient
download error.  This also switches to consistent use of wget
throughout, which reports download errors more cleanly in the travis
build logs.
Rename travis "arm" BUILD_ARCH to "arm-apcs".

These platforms are built with an old ARM compiler that supports the
deprecated APCS standard.
Increase fixed filename sizes in SLIP tunnels

Long serial port names like
  /dev/serial/by-id/usb-FTDI_FT230X_Basic_UART_DN0038W8-if00-port0
cause crashes.  This is the simplest fix to avoid the problem.
Add ADuCRF101 library code from Analog Devices
Add CMSIS library code from ARM
Clean up ADuCRF101 library code for Contiki use
Add initial ADuCRF101 CPU support
Add initial EV-ADuCRF101MKxZ platform support
Add ADuCRF101 radio driver
Fix ADI radio engine driver and add hook for packet reception
Move RF RX into its own process
Add button sensor support (BOOT button)
Add platform LED driver
Implement rtimers for ADuCRF101
Add dummy uart0/uart1 headers that some examples require
Add sniffer support to ADuCRF101

This works with the examples/cc2538dk/sniffer/ project and the
corresponding "sensniff" host-side tool.
Add README.md for ev-aducrf101mkxz platform
Add IAR compiler support for ADuCRF101
Add Windows build instructions for ev-aducrf101mkxz platform
Fix radio hangups when trying to transmit with radio off.

The border-router tries to transmit and do other stuff after turning
the radio off, and the radio driver didn't handle that very well.
With this fix, it's no longer necessary to reset the border router
after starting tunslip6.