Allow alternate implementation of GCM

Provide the ability to use an alternative implementation of GCM in place
of the library-provided implementation.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 227faed..d383308 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Features
+   * Add support for alternative implementations of GCM, selected by the
+     configuration flag MBEDTLS_GCM_ALT in config.h
+
 = mbed TLS 2.6.0 branch released 2017-08-10
 
 Security