fs: build host-mkpasswd when BR2_ROOTFS_USERS_TABLES is used

The code in fs/common.mk properly triggers the build of host-mkpasswd
when at least one package specifies a <pkg>_USERS variable. However,
when no selected package specifies a <pkg>_USERS variable but the user
uses a custom users table through BR2_ROOTFS_USERS_TABLES, then we
forget to build host-mkpasswd, leading to build failures if you don't
have mkpasswd installed in your machine.

This commit fixes that by taking into account both the package users
table and the custom users table when deciding to depend or not on
host-mkpasswd.

Reported-by: Auke Willem Oosterhoff <oosterhoff@baopt.nl>
Cc: Auke Willem Oosterhoff <oosterhoff@baopt.nl>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>