Makefile: cleanup linking to librttest.a

Only add '-lrttest -L.' where it's actually needed.
Use '$<' instead of '$^'. Otherwise librttest is added twice:
As 'librttest.a' and as '-lrttest'.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: John Kacur <jkacur@redhat.com>