Fixes time() abstraction for custom configs

Added platform abstraction of time() to ChangeLog, version features, and fixed the build for dynamic configuration.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 1281138..3e2ea6b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,10 @@
 
 = mbed TLS 2.x branch
 
+Features
+   * Support for platform abstraction of the standard C library time()
+     function.
+
 Bugfix
    * Fix bug in mbedtls_mpi_add_mpi() that caused wrong results when the three
      arguments where the same (in-place doubling). Found and fixed by Janos