build: Allow repeated reinstalls

Add -f to $(LN_S) so that we may overwrite the symlink created from a
previous install.  This allows one to reinstall the code without first
manually removing the existing symlink.

Change-Id: Ia7597e6b206975ead0f0a021c5288061f2798123
Signed-off-by: Adam Litke <alitke@redhat.com>