initscripts: new package

The folder init.d is currently installed by default since it's part of
our skeleton.
This patch creates a package out of it and make busybox/sysvinit depends
on it.

This way, if you chose another init, you don't end up with a useless
init.d folder.

[Thomas:
  - make the initscripts package selectable via a hidden bool
  - remove some unneeded changes in sysvinit.mk.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>