dropbear: add options to disable wtmp / lastlog logging

Buildroot does not have wtmp or lastlog support by default so allow dropbear
to be configured to stop using them and hence to stop generating system log
error messages.

[Peter: invert to positive options, and disable by default]
Signed-off-by: Will Moore <will.moore@beraninstruments.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>