tests/boot-serial-test: Fix problem with timeout due to dropped characters

tests/boot-serial-test: Fix problem with timeout due to dropped characters

Commit 92b540dac9fc3a5 introduce a counter to handle the timeouts in a
better way. But in case ccnt reaches 512, the current read character is
ignored - and if that character is part of the string that we are looking
for, the test fails to match the string.

Almost all of the tests look for a string within the first 512 bytes of
firmware output, so the problem never triggered there. But the hppa test
that has been added recently looks for a longer string at the very end of
a long output, thus there's a chance that we miss a character there so
that the test fails unexpectedly. Fix it by *not* reading and dropping a
character if the counter reaches 512.

Fixes: 92b540dac9fc3a572c7342edd0b073000f5a6abf
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1518761564-9899-1-git-send-email-thuth@redhat.com
[PMM: added initializer for nbd to silence false-positive warning
 from OpenBSD 6 compiler]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>