Add retain basic implementation on GNU/Linux

Retain is saved in file and backup of this file is done every time.
File isn't saved on every PLC cycle to avoid high I/O load.
By default, retain is saved every second and on PLC stop/shutdown.
It's possible to override retain flush period by using CFLAGS option
in project's settings. For example, for 0.5 seconds period
use "-DFILE_RETAIN_SAVE_PERIOD_S=0.5".

     1

<?xml version='1.0' encoding='utf-8'?>

     2

<BeremizRoot URI_location="LOCAL://">

     3

  <TargetType/>

     4

</BeremizRoot>