dropbear: disable standalone mode for nommu

dropbear for nommu targets must disable the standalone (non-inetd) mode
since the platform lacks fork(), fixes:
http://autobuild.buildroot.net/results/ecbdfab10f54bec75c112bbf005bba9ae2b18dc8/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>