Add support for alternative ECJPAKE implementation

This commit allows users to provide alternative implementations of the
ECJPAKE interface through the configuration option MBEDTLS_ECJPAKE_ALT.
When set, the user must add `ecjpake_alt.h` declaring the same
interface as `ecjpake.h`, as well as add some compilation unit which
implements the functionality. This is in line with the preexisting
support for alternative implementations of other modules.
diff --git a/ChangeLog b/ChangeLog
index a200d51..0b8667b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -34,6 +34,8 @@
      The following functions from the ECDH module can be replaced
      with an alternative implementation:
      mbedtls_ecdh_gen_public() and mbedtls_ecdh_compute_shared().
+   * Add support for alternative implementation for ECJPAKE, controlled by
+     new configuration flag MBEDTLS_ECJPAKE_ALT.
 
 API Changes
    * Extend RSA interface by multiple functions allowing structure-