package/linux-tools: add support for installing init system files

package/linux-tools: add support for installing init system files

Some linux tools (e.g. the Microsoft HyperV convenience utilities) will
install programs tostart at boot time, so they need to be able to
install init files (systemd units, sysv init script, or openrc units).

Unlike the other commands, we are redefining the real _INSTALL_INIT_*
macros, rather than use hooks, to let the infra call those at the right
moment.

We must be careful about the openrc support, though: if two tools are
enabled, one which provides sysv scripts but no openrc config, and the
other which provides openrc config, and we are using openrc as init
system, then we want to use the sysv scripts from the former as well as
the openrc config of the latter. Thus we need to duplicate a bit the
openrc logic here.

Signed-off-by: Pascal de Bruijn <p.debruijn@unilogic.nl>
[yann.morin.1998@free.fr:
  - define macros, not hooks
  - introduce support for openrc too
  - expand commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>