Rename time and index parameter to avoid name conflict.

As noted in #557, several functions use 'index' resp. 'time'
as parameter names in their declaration and/or definition, causing name
conflicts with the functions in the C standard library of the same
name some compilers warn about.

This commit renames the arguments accordingly.
diff --git a/ChangeLog b/ChangeLog
index 6bf2a5a..fb8d321 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -172,6 +172,8 @@
      The PK and RSA modules use different types for passing hash length and
      without these checks the type cast could lead to data loss. Found by Guido
      Vranken.
+   * Avoid shadowing of time and index functions through mbed TLS function
+     arguments. Found by inestlerode. Fixes #557.
 
 = mbed TLS 2.4.2 branch released 2017-03-08