Fix -Wshadow warnings

Backport of ea35666 and 824ba72

see #240
diff --git a/ChangeLog b/ChangeLog
index 43dd6b2..2c369ce 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -8,6 +8,7 @@
    * Fix bug in mbedtls_rsa_public() and mbedtls_rsa_private() that could
      result trying to unlock an unlocked mutex on invalid input (found by
      Fredrik Axelsson) (#257)
+   * Fix -Wshadow warnings (found by hnrkp) (#240)
 
 = mbed TLS 1.3.12 released 2015-08-11