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.