busybox: adjust nommu swaponoff handling for busybox 1.26.0+

busybox: adjust nommu swaponoff handling for busybox 1.26.0+

Fixes:
http://autobuild.buildroot.net/results/dfe/dfec2c0626ab087325fd869bcbe0b9dc354d788c/

Busybox 1.26.0 gained separate config options for swapon / swapoff:

commit f417ffd88f194bbfd18605882ee242190c1bab34
Author: Denys Vlasenko <vda.linux@googlemail.com>
Date:   Mon Nov 14 17:30:50 2016 +0100

    Make swapon and swapoff individually selectable.

    For example, without swapoff, code shrinks by 277 bytes.

    Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
So adjust busybox.config and the logic to disable these applets for nommu
builds for the new names.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>