busybox: adjust configuration for non-MMU targets

The swaponoff applet doesn't build (and doesn't make sense) on non-MMU
platforms.

The ash shell cannot build on non-MMU platforms (because it uses
fork()), so select the hush shell instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>