net-snmp: have mib-modules=smux enabled

According to the following commit, net-snmp needs to have
mib-modules=smux enabled:
commit f64db301


Author: Aws Ismail <aws.ismail@windriver.com>
Date:   Mon Mar 25 11:30:06 2013 -0400

    Quagga has no snmp support, unrecognized options --enable-tcp-md5

    1. Quagga's tcp-md5 has been renamed to linux24-tcp-md5

    2. net-snmp needs to have mib-modules=smux enabled to enable
    quagga to support snmp. Make the net-snmp option dependent
    on the DISTRO_FEATURE snmp.

    3. Misc: install the sample conf files for quagga. Also,
    Make sure that the post install script is being run
    on the target rather than during the rootfs creation
    stage.

Signed-off-by: Amy Fong <amy.fong@windriver.com>
Signed-off-by: Aws Ismail <aws.ismail@windriver.com>
Signed-off-by: Joe MacDonald <joe.macdonald@windriver.com>

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>