baylibre-acme: Use timerfd instead of a fake pipe.

Currently baylibre-acme uses a fake pipe as the input channel required by
libsigrok API and calls sleep() in the data acquisition callback to create
intervals between measurements.

Switch to a more elegant approach: use Linux' timerfd and set a periodic
timer equal to the sampling rate. Then read the data every time the timer
expires.

Signed-off-by: Daniel Lezcano <redacted>
Signed-off-by: Bartosz Golaszewski <redacted>