ECDH: Add ChangeLog for legacy buffer overflow

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

ChangeLog.d/ecdh-legacy-buffer-overflow.txt

@@ -0,0 +1,7 @@
+Security
+   * Possible buffer overflow in mbedtls_ecdh_calc_secret(): when the provided
+     output buffer is too small, sometimes (depending on the value of the
+     computed shared secret), the function would not return an error as it
+     should, but instead write past the end of the buffer, with consequences
+     that may range up to arbitrary code execution depending on what follows the
+     buffer. Reported by Eva Crystal (0xiviel) from XSource Security.