SinqS7PLCFW

This is a fork of Dirk Zimoch's epics_driver_modules/s7plcFW, with the following adjustments:

• The String offset, now takes the two prefix bytes (max string length, current string length) into account, so you don't have to add 2 to each offset in order for the driver to read the correct string
• The length of a String read from the SPS and written to the Val field of a stringin record is no longer static, but instead read from the SPS directly, to avoid having bytes leftover in the buffer from a previous string.
• The Makefile has been slightly adjusted, limiting the build targets.

The default driver behaviour is documented in the file s7plcFW.html.