netsnmp: Add option for using minimal internal SSL code

Add option for enabling a minimal internal copy of OpenSSL usable for USM
security. It will not enable the usage of SNMP over (D)TLS.

[Thomas: use 'else ifeq' to avoid having another nested ifeq ... endif
block.]

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>