external/ppp: update for BoringSSL.

This change updates the Android specific changes for BoringSSL, which is
Google's cleaned up version of OpenSSL. The change also works with the
current OpenSSL in Android.

Significant changes:
  * Removal of wrapper functions for the OpenSSL hash functions – these
    aren't needed.
  * Use of the OpenSSL DES API. The code currently uses the SSLeay DES
    functions which are supported by current OpenSSL but were deprecated
    sometime in the 1990's. The OpenSSL DES API is the supported API for
    both OpenSSL and the only API in BoringSSL.

    This change has been proposed upstream in
    https://github.com/paulusmack/ppp/pull/24.

Change-Id: I06c87ae4b5175a268739300eab59d65ddac8a2cc
Merge "external/ppp: update for BoringSSL."