rtapi/Submakefile separate build hack:  mask race condition with ln -s

Running make -j2, a race condition exists between checking and
creating the hack symlink.  Mask this effect by using ln -sf, which
instead of failing, will simply overwrite the link if it already
exists.  Hey, it's a hack, right?
Makefile:  Simplify & prettify kbuild invocation (should be no-op)

- Wrap long output lines
- Don't cd, just use make -C