Restore P>Q in RSA key generation (#558)

The PKCS#1 standard says nothing about the relation between P and Q
but many libraries guarantee P>Q and mbed TLS did so too in earlier
versions.

This commit restores this behaviour.
diff --git a/ChangeLog b/ChangeLog
index 5270685..36dc90e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -21,6 +21,7 @@
      by inestlerode. #559.
    * Fix documentation and implementation missmatch for function arguments of
      mbedtls_gcm_finish(). Found by cmiatpaar. #602
+   * Guarantee that P>Q at RSA key generation. #558
 
 = mbed TLS 2.1.5 branch released 2016-06-28