netsnmp: configurable MIB modules

Make the MIB modules, which are included/excluded in the build, to be
configurable instead of being hard coded.

[Peter: drop menuconfig]
Signed-off-by: przemyslaw <przemyslaw.wrzos@calyptech.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>