Fix #ifdef inconsistency

fixes #310

Actually all key exchanges that use a certificate use signatures too, and
there is no key exchange that uses signatures but no cert, so merge those two
flags.

Conflicts:
	ChangeLog
diff --git a/ChangeLog b/ChangeLog
index 0712001..5ebd748 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,12 +5,11 @@
 Changes
    * Improved performance of mbedtls_ecp_muladd() when one of the scalars is 1
      or -1.
-
-= mbed TLS 2.1.2 released 2015-10-06
-
 Bugfix
    * Fix build error with configurations where ECDHE-PSK is the only key
      exchange. Found and fix provided by Chris Hammond. #270
+   * Fix build error with configurations where RSA, RSA-PSK, ECDH-RSA or
+     ECHD-ECDSA if the only key exchange. Multiple reports. #310
 
 = mbed TLS 2.1.2 released 2015-10-06