Abstracts away time()/stdlib.h into platform

Substitutes time() into a configurable platform interface to allow it to be
easily substituted.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 3e05f3f..67c62b7 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -542,7 +542,7 @@
 struct mbedtls_ssl_session
 {
 #if defined(MBEDTLS_HAVE_TIME)
-    time_t start;               /*!< starting time      */
+    mbedtls_time_t start;       /*!< starting time      */
 #endif
     int ciphersuite;            /*!< chosen ciphersuite */
     int compression;            /*!< chosen compression */