system: move remounting / to the corresponding init systems

system: move remounting / to the corresponding init systems

Currently, remounting / read-write (or not) is done by the skeleton
package when the init system is either busybox or sysvinit, by
registering a target-finalize hook; it is not done at all for systemd.

Move registering this target-finalize hook to both of busybox and
sysvinit. Leave systemd alone, we'll take care of it later.

Rename the macro to a more meaningful name, and move it to system.mk
with the other such macros.

Yet a little bit less init-system knowledge in the skeleton.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: remove not-so-useful comments, as pointed by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>