libsrtp: new package

[Thomas:
 - Add better Config.in help text and fix its indentation
 - Fix comment header in .mk file
 - Uncomment useful commented lines in .mk file (SITE and AUTORECONF)
 - Add LICENSE_FILES variable.
 - Add handling of static/shared libraries build/installation.
 - Add dependency on host-pkgconf, so that libsrtp installs its own
   .pc file.
 - Add optional dependency on OpenSSL.
 - Use github helper.
 - Remove hash file, since we're downloading from Git.]

Signed-off-by: Gregory Dymarek <gregd72002@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>