initscripts: check if swapon/swapoff exists before executing them

initscripts: check if swapon/swapoff exists before executing them

Not all built images contain swapon/swapoff, for instance, it is
configurable with or without them in busybox. So it'd better to check if
they exist or not before executing them.

Redirecting the potential errors to /dev/null is not good enough, which
might suppress the *real* errors.

Signed-off-by: Ming Liu <peter.x.liu@external.atlascopco.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>