use CLOCK_MONOTONIC instead of CLOCK_REALTIME for timer setup

CLOCK_REALTIME is a wall clock, that could change back and forth (user
changes system time, winter/summer time, NTP corrections and so on).
For real-time application CLOCK_MONOTONIC should be used, it doesn't
have such jumps.

Beremiz's user manual
=====================

Contents:

.. toctree::
   :maxdepth: 2

   install
   start
   edit
   build
   connectors
   debug