Defines MBEDTLS_ECDH_LEGACY_CONTEXT in ecdh.h

Commit adds the conditional definition of
MBEDTLS_ECDH_LEGACY_CONTEXT to ecdh.h.
MBEDTLS_ECDH_LEGACY_CONTEXT is only defined
if MBEDTLS_ECP_RESTARTABLE is definied.

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>

include/mbedtls/ecdh.h

@@ -40,6 +40,39 @@
 
 #include "mbedtls/ecp.h"
 
+/**
+ * \def MBEDTLS_ECDH_LEGACY_CONTEXT
+ *
+ * Use a backward compatible ECDH context.
+ *
+ * Mbed TLS supports two formats for ECDH contexts (#mbedtls_ecdh_context
+ * defined in `ecdh.h`). For most applications, the choice of format makes
+ * no difference, since all library functions can work with either format,
+ * except that the new format is incompatible with MBEDTLS_ECP_RESTARTABLE.
+
+ * The new format used when this option is disabled is smaller
+ * (56 bytes on a 32-bit platform). In future versions of the library, it
+ * will support alternative implementations of ECDH operations.
+ * The new format is incompatible with applications that access
+ * context fields directly and with restartable ECP operations.
+ *
+ * Define this macro if you enable MBEDTLS_ECP_RESTARTABLE or if you
+ * want to access ECDH context fields directly. Otherwise you should
+ * comment out this macro definition.
+ *
+ * This option has no effect if #MBEDTLS_ECDH_C is not enabled.
+ *
+ * \note This configuration option is experimental. Future versions of the
+ *       library may modify the way the ECDH context layout is configured
+ *       and may modify the layout of the new context type.
+ */
+
+#if defined(MBEDTLS_ECP_RESTARTABLE)
+#define MBEDTLS_ECDH_LEGACY_CONTEXT
+#else
+#undef MBEDTLS_ECDH_LEGACY_CONTEXT
+#endif
+
 #if defined(MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED)
 #undef MBEDTLS_ECDH_LEGACY_CONTEXT
 #include "everest/everest.h"