busybox: add an inittab entry to activate swap

There is a call to swapoff in the shutdown sequence, so call "swapon -a"
on startup. As stated in the swapon man page,

   All devices marked as "swap" in /etc/fstab are made available, except
   for those with the "noauto" option. Devices that are already being
   used as swap are silently skipped.

So even if the system has some init script to start/stop swap (e.g. from
a rootfs ovelay) calling swapon/swapoff would be harmless.

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit d524cc7d9de549228b998ba5747b0d7cd4f24251)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/busybox/inittab

@@ -18,6 +18,7 @@
 ::sysinit:/bin/mount -o remount,rw /
 ::sysinit:/bin/mkdir -p /dev/pts /dev/shm
 ::sysinit:/bin/mount -a
+::sysinit:/sbin/swapon -a
 null::sysinit:/bin/ln -sf /proc/self/fd /dev/fd
 null::sysinit:/bin/ln -sf /proc/self/fd/0 /dev/stdin
 null::sysinit:/bin/ln -sf /proc/self/fd/1 /dev/stdout