initramfs: fix init symlink creation

The -e test will dereference the symlink, so if there is no /bin/init,
we will constantly try to create the symlink.  So rather than error on
subsequent runs when the link exists, use the force flag to ln.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>