package/skeleton: drop dependency on host-mkpasswd

Setting the root pasword is done in a target-finalize hook, so we do not
need to enforce a dependency from the skeleton onto host-mkpasswd.

Dropping that dependency will simplify making skeleton a virtual
package (in up-coming changes).

Instead, it is now selected as any other package. As such, it is
guaranteed to be built before target-finalize.

This however introduces a slight change in behaviour: previously,
host-mkpasswd would only be built if we needed to hash the root password
from its plain-text value. Now, host-mkpasswd is always built as soon as
the root password is non-empty, even if already pre-hashed.

Since host-mkpasswd is a really tiny weeny package bundled in Buildroot,
with only two C files, built as a single unit with a single gcc call,
the overhead is really minimal. Compared to the simplifications this
will allow in the skeleton packages (plural: common, sysv, systemd,
custom) to come, this overhead is acceptable.

Yet another simplification, even if small, to ease providing multiple
skeletons.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>