Bugfix: Heisenbug when parsing console output in entropy check

[mbuechse]

Resolves #1007.

The login prompt generated by the serial getty service could mess up the output and throw off our primitive parser.

Thanks @garloff for the crucial tip!

[mbuechse]

I have tested this code. I added debug output for the whole console output, and the login prompt no longer appears in multiple attempts. I am pretty confident that the problem is gone or at least greatly reduced...

[garloff]

I cannot judge for sure whether this provides a 100% guarantee to solve the issue or there is a possible race left where systemd has already invoked the getty process that is in the scheduling queue and that outputs the login prompt still intermingled with the debug output. Maybe it's impossible, maybe it's not. In the latter case, it's for sure very unlikely, and as our lives do not depend on it, I'd judge this good enough. Time will tell if we still have <0.1% fails left or not. If so, we can deal with it then. For sure, this is a huge improvement.