vim: install /bin/vi as a relative symlink

Prevent creating a dangling symlink when vim is not present on the host
machine. With BR2_ROOTFS_MERGED_USR, just link to "vim", since they are
on the same directory, otherwise link to "../usr/bin/vim".

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>