dahdi_genconf: only generate system.conf

Run dahdi_genconf with the option 'system' to only use the System
(/etc/dahdi/system.conf) generator.

No point in generating /etc/asterisk/dahdi-channels.conf and (in some
versions) /etc/dahdi/span-types.conf and /etc/dahdi/assigned-spans.conf .
Needlessly generating the above two can break user's configuration.

Signed-off-by: Andrew Nagy <andrew.nagy@the159.com>