fs/common.mk: add host-mkpasswd if PACKAGES_USERS is not empty

When using a custom rootfs skeleton, the host-mkpasswd target is not
automatically built, even if some packages add users (i.e. when
PACKAGES_USERS is not empty).

In this case, the rootfs generation may fail because the mkpasswd
provided by some distribution does not support all options used in the
mkusers script.

This patch avoids such trouble by automatically adding the host-mkpasswd
package to the dependency list of the rootfs targets if users needs
to be created.

[Peter: minor rewording of commit message]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>