package/netopeer2: add package

Netopeer2 is a set of tools implementing network configuration
tools based on the NETCONF Protocol.

Netopeer2 needs libnetconf2 to have SSL/TSL and SSH support, so
we enable both openssl and libssh+server from netopeer2, so that
libnetconf2 has appropriate support. But netopeer2 does not use
either, so does not build-depend on them.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
[yann.morin.1998@free.fr:
  - fix dependencies and their comments
  - explain openssl and libssh+server dependencies
  - fix codestyle in Config,in, noticed by Adam
  - fix codestyle in .mk
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>