rtcds: read in environment before setting flags and change flag tests to explictly look for non-empty strings.

The entire block of code for initializing environment variables has been moved before the command line flag checks.
The USERLAND_FLAG and NO_KERNELSPACE_FLAG default values are determined by the environment, but the /etc/advligorts files need to be read in first.

Command line flags are cleared if empty string, or set if not empty.  This wasn't obvious originally since the test for flags was
just [[ $SOME_FLAG ]].

The tests have been changed to [[ -n $SOME_FLAG ]] or in the negative case [[ -z $SOME_FLAG ]].
This doesn't change the behavior but is less ambigous to the eye.