systemd: include sysvinit in default PACKAGECONFIG only if in DISTRO_FEATURES

The sysvinit PACKAGECONFIG knob enables various legacy/compatibility
code that may not be needed or even desired. If DISTRO_FEATURES
includes systemd (as it must for this recipe to build) but not
sysvinit, there is no point building and installing that legacy
support.

As most other changes, this can cause breakage, but given that
sysvinit not being in DISTRO_FEATURES requires explicit opt-out (due
to backfill), I think the risk is low. Moreover, it is generally
easier to add to than to remove from PACKAGECONFIG.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>