ECP: Add module and function level replacement options.

include/mbedtls/ecp.h

@@ -37,6 +37,10 @@
 #define MBEDTLS_ERR_ECP_INVALID_KEY                       -0x4C80  /**< Invalid private or public key. */
 #define MBEDTLS_ERR_ECP_SIG_LEN_MISMATCH                  -0x4C00  /**< Signature is valid but shorter than the user-supplied length. */
 
+#if !defined(MBEDTLS_ECP_ALT)
+// Regular implementation
+//
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -654,16 +658,22 @@ int mbedtls_ecp_gen_key( mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
 int mbedtls_ecp_check_pub_priv( const mbedtls_ecp_keypair *pub, const mbedtls_ecp_keypair *prv );
 
 #if defined(MBEDTLS_SELF_TEST)
+
 /**
  * \brief          Checkup routine
  *
  * \return         0 if successful, or 1 if a test failed
  */
 int mbedtls_ecp_self_test( int verbose );
-#endif
+
+#endif // MBEDTLS_SELF_TEST
 
 #ifdef __cplusplus
 }
 #endif
 
+#else  /* MBEDTLS_ECP_ALT */
+#include "ecp_alt.h"
+#endif /* MBEDTLS_ECP_ALT */
+
 #endif /* ecp.h */