sysvinit: update the inittab to support "single" from the kernel

sysvinit: update the inittab to support "single" from the kernel

Closes #8911

When the kernel passes single in the command line, this translates
into an init -s option that is suppose to drop into a shell after
the sysinit and before the runlevel.

So, in busybox this is hardcoded - but, in sysvinit using the
sysinit action for the rcS means that it will always be executed
even when trying to get into single user mode for repair.

This change should run rcS in all of the expected runlevels
1-5 and should achieve the desired result compared to the busybox
and still allow a single user shell to get started as expected
before running rcS.

Signed-off-by: Charles Hardin <ckhardin@exablox.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>