Add HTTPS support using BoringSSL.

Upstream implements HTTPS using GnuTLS but BoringSSL is preferred. This
CL removes all use of GnuTLS in src/microhttpd and implements HTTPS with
BoringSSL.

Bug: 23153045
Change-Id: Ibbe17e5e7b87fee32832bda30591f93ff023a807
7 files changed