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.

include/mbedtls/config.h

@@ -271,6 +271,7 @@
 //#define MBEDTLS_CMAC_ALT
 //#define MBEDTLS_DES_ALT
 //#define MBEDTLS_DHM_ALT
+//#define MBEDTLS_ECJPAKE_ALT
 //#define MBEDTLS_GCM_ALT
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT

include/mbedtls/ecjpake.h

@@ -44,6 +44,8 @@
 #include "ecp.h"
 #include "md.h"
 
+#if !defined(MBEDTLS_ECJPAKE_ALT)
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -223,17 +225,31 @@ int mbedtls_ecjpake_derive_secret( mbedtls_ecjpake_context *ctx,
  */
 void mbedtls_ecjpake_free( mbedtls_ecjpake_context *ctx );
 
+#ifdef __cplusplus
+}
+#endif
+
+#else  /* MBEDTLS_ECJPAKE_ALT */
+#include "ecjpake_alt.h"
+#endif /* MBEDTLS_ECJPAKE_ALT */
+
 #if defined(MBEDTLS_SELF_TEST)
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * \brief          Checkup routine
  *
  * \return         0 if successful, or 1 if a test failed
  */
 int mbedtls_ecjpake_self_test( int verbose );
-#endif
 
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #endif /* ecjpake.h */