Rework of the init system

Since we have now two uncompatible init systems, and we want only one of
them at the same time in use in the rootfs, we need to select a
particular init system. This patch also adds $(PKG)_INSTALL_INIT_SYSTEMD
and $(PKG)_INSTALL_INIT_SYSV hooks that are called when the matching
init systems are selected to install properly the init scripts of the
package.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>