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/config.h b/include/mbedtls/config.h
index a617d06..3c3d9ca 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -152,6 +152,7 @@
  * platform function
  */
 //#define MBEDTLS_PLATFORM_EXIT_ALT
+//#define MBEDTLS_PLATFORM_TIME_ALT
 //#define MBEDTLS_PLATFORM_FPRINTF_ALT
 //#define MBEDTLS_PLATFORM_PRINTF_ALT
 //#define MBEDTLS_PLATFORM_SNPRINTF_ALT
@@ -2465,6 +2466,7 @@
 //#define MBEDTLS_PLATFORM_STD_CALLOC        calloc /**< Default allocator to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_FREE            free /**< Default free to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_EXIT            exit /**< Default exit to use, can be undefined */
+//#define MBEDTLS_PLATFORM_STD_TIME            time /**< Default time to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_FPRINTF      fprintf /**< Default fprintf to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_PRINTF        printf /**< Default printf to use, can be undefined */
 /* Note: your snprintf must correclty zero-terminate the buffer! */