long lines confuse PHD's serial listener

[iximeow]

at least, it's extremely likely it's carriage returns that come with long lines, something to that effect.

#756 landed with this kind of hokey workround: when writing a very long line to a guest, PHD would appear to hang. it's certainly that PHD is waiting for the guest to echo back what it wrote, but the guest wrote \r\n and the expected character is just a bit beyond that.

whatever it is, this really ought to be fixed in PHD, rather than a one-off helper in boot_order.rs.