snmppp: add option to disable SNMPv3 support

Disabling SNMPv3 support also removes the dependency on OpenSSL, which is
pretty large (over 2 MB of uncompressed filesystem size on an ARM926 platform).

BR2_PACKAGE_SNMPPP_SNMPV3 defaults to yes for backward compatibility with
previous Buildroot releases, where SNMPv3 was always enabled.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>