spkmodem-decode: edge detection *and* amplitude

for pulses, we currently use amplitude detection.

edge detection is better, because weak / low gain
signals will be more reliable. if audio is coming
in on/from a system that does automatic gain
adjustment, this once again is more robust too.

microphones and speakers (which people often use
with spkmodem if nothing else available) often
clamp amplitude, to an extent that this software
may not detect those pulses reliably that way.

so we detect slope edges instead. this causes
very little performance penalty (use of abs(),
that's about it)

however, edge detection is inherently vulnerable
to noise, so we will also detect amplitude. this
acts as an effective noise filter, while still
improving pulse detection.

Signed-off-by: Leah Rowe <leah@libreboot.org>

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