system: fix BR2_INIT_NONE / BR2_INIT_SYSTEMD build

Following the introduction of the initscripts package in commit
89d39fc7 ("initscripts: new package"),
the /etc/inittab file is no longer part of the skeleton, and therefore
is not always installed.

However, system/system.mk tweaks the inittab for getty and filesystem
remount as rw, without taking the precautions of whether the init
system is Busybox or SysV. This commit fixes that by adding the
necessary conditions around the code adjusting the inittab file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>