Remove /rtlib directory from git, since it's really just a build artifact
universal build:  enable a single kthreads universal build

Mainly, move final output from ../rtlib to ../rtlib/<flavor>
- Makefile:  create RTLIBDIR for kbuild flavors
- halcmd_commands:  update module location code with rtapi_compat functions
- rtapi.conf:  append $FLAVOR to generic EMC2_RTLIB_DIR

Also fix RTFLAGS in kbuild
universal build:  build kthreads modules in a separate tree

HACK ALERT

It looks almost impossible to coerce kbuild into putting build
artifacts outside the source tree.  The universal build needs to
keep these artifacts separate for each kthread flavor.

To solve this, create a link tree in objects/rtapi-<flavor> with
everything needed to build kernel modules.

In the 'modules:' target, cd to the link tree and call kbuild just as
normally done from the src/ directory.
universal build:  fix make setuid for all flavors