Move OpenSSLConfig.cmake into the cmake/ directory

We've already put perlasm.cmake in there. I figure CMake helper files
can go in there. It also seems to match what other projects too.

Change-Id: Ief6b10cf4e80b8d4b52ca53b90aa425b32037e52
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/56566
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

CMakeLists.txt

@@ -611,4 +611,4 @@ install(EXPORT OpenSSLTargets
   NAMESPACE OpenSSL::
   DESTINATION lib/cmake/OpenSSL
 )
-install(FILES OpenSSLConfig.cmake DESTINATION lib/cmake/OpenSSL)
+install(FILES cmake/OpenSSLConfig.cmake DESTINATION lib/cmake/OpenSSL)