libsrtp: use OpenSSL

libsrtp: use OpenSSL

Use OpenSSL instead of the internal crypto backend. Everything in
OpenWrt that links to libsrtp2 depends on OpenSSL anyway.

Upsides:

  - the libsrtp2 package size shrinks a bit (for example from 35 to 24
    KiB on ath79)
  - allows to use more cipher suites
  - may allow for hardware acceleration

Closes #763

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>