res/res_rtp_asterisk: Add ECDH support

This will add ECDH support to Asterisk. It will
detect auto ECDH support in OpenSSL
(1.0.2b and above) during ./configure. If this is
available, it will use it,
otherwise it will fall back to prime256v1 (this
behavior is consistent with
other projects such as Apache and nginx).

This fixes WebRTC being broken in Firefox 38+ due
to Firefox now only supporting
ciphers with perfect forward secrecy.

ASTERISK-25265 #close

Change-Id: I8c13b33a2a79c0bde2e69e4ba6afa5ab9351465b