sam-ba: specify ln -sf to remove existing destination file

This fixes below build error when executing
"make host-sam-ba-dirclean; make host-sam-ba-rebuild".

ln -s ../../opt/sam-ba/sam-ba /opt/test/br-x86/buildroot/output/host/usr/bin/sam-ba
ln: failed to create symbolic link '/opt/test/br-x86/buildroot/output/host/usr/bin/sam-ba': File exists
make: *** [/opt/test/br-x86/buildroot/output/build/host-sam-ba-2.12/.stamp_host_installed] Error 1

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>