'e' emulator should have a headless option to facilitate Travis-based CI.

[neuschaefer]

A few ideas:

• BYE or a different forth word could turn off the interpreter (see also src/e/processor.c, do_hostif)
• Running the forth interpreter with UART input/output may also be useful on hardware, and I think the same firmware should run on hardware and in e.
• The "stdio" device could be selected through a GPIO pin that's read when Forth starts. This pin could can simulated in e (obviously), and hooked up to a jumper header or DIP switch on hardware. This is a simple configuration mechanism, but won't scale to many bits of configuration.
• The LEDS line interferes with the UART, at least when stdout and stderr are connected to the same terminal.

[sam-falvo]

No longer relevant; new computer specification is already headless (uses serial port for communication with outside world), so 'e' no longer represents a real machine.