package/skeleton-init-sysv: conditionally enable swapon/swapoff in inittab

package/skeleton-init-sysv: conditionally enable swapon/swapoff in inittab

The default inittab files added by busybox and sysvinit runs 'swapon -a'
during init and 'swapoff -a' during shutdown, but those programs are not
guaranteed to be available, so the boot log may become polluted by error
messages like this:

    swapon: not found

Add a target-finalize hook to skeleton-init-sysv that enables or disables
the swapon/swapoff lines in /etc/inittab, depending on the existence of
$(TARGET_DIR)/sbin/swap{on,off}.

Based on a previous patch sent by Thomas De Schampheleire.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>