Minor modifications for alt support in des and ecp

1. Add 3des context to be allowed for alternative defintion
2. Move some ecp structs, to disallow alternative definition of them,
as other modules rely on them

include/mbedtls/des.h

@@ -67,10 +67,6 @@ typedef struct
 }
 mbedtls_des_context;
 
-#else  /* MBEDTLS_DES_ALT */
-#include "des_alt.h"
-#endif /* MBEDTLS_DES_ALT */
-
 /**
  * \brief          Triple-DES context structure
  */
@@ -80,6 +76,10 @@ typedef struct
 }
 mbedtls_des3_context;
 
+#else  /* MBEDTLS_DES_ALT */
+#include "des_alt.h"
+#endif /* MBEDTLS_DES_ALT */
+
 /**
  * \brief          Initialize DES context
  *