scan-for-best-tpr3.rb: An optional cmdline argument for text description

Now the second command line argument can be used to provide a text
description of this test run. For example, the exact information
about the device and voltage tweaks. Or anything else that is not
normally reported by a10-meminfo.
A new tool: a10-stdin-watchdog

This is a variant of a watchdog program, which allows to reset
the timeout counter by providing updated timeout values via stdin.
a10-stdin-watchdog: Implement a simplistic built-in memtester

In the case of a fatal dram failure, the watchdog itself may have
troubles with its timeout counter corruption and similar bad things.
So additionally implement a trivial memtester functionality to make
it a little bit more robust.
Use a10-stdin-watchdog in scan-for-best-tpr3.rb

This should be a more reliable use of the hardware watchdog.
scan-for-best-tpr3.rb: Nicer error reporting about missing prerequisites