sysvinit: install new cmds and fix inittab

Adding support for sysvinit to install reboot, poweroff, and pidof
commands available in /sbin.

Fixes bug #6620

Removed uninstall commands since they aren't used.

There is an error with sysvinit that will prevent the system from booting
if "dshm::sysinit:/bin/mkdir -p /dev/shm" is not present in the inittab.

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