[crypto] Expose the base point as an explicit elliptic curve property

Add the generator base point as an explicit property of an elliptic
curve, and remove the ability to pass a NULL to elliptic_multiply() to
imply the use of the generator base point.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/crypto.h

@@ -181,9 +181,11 @@ struct elliptic_curve {
 	size_t pointsize;
 	/** Scalar (and private key) size */
 	size_t keysize;
+	/** Generator base point */
+	const void *base;
 	/** Multiply scalar by curve point
 	 *
-	 * @v base		Base point (or NULL to use generator)
+	 * @v base		Base point
 	 * @v scalar		Scalar multiple
 	 * @v result		Result point to fill in
 	 * @ret rc		Return status code

src/include/ipxe/weierstrass.h

@@ -160,6 +160,7 @@ extern int weierstrass_multiply ( struct weierstrass_curve *curve,
 		.name = #_name,						\
 		.pointsize = ( WEIERSTRASS_AXES * (_len) ),		\
 		.keysize = (_len),					\
+		.base = (_base),					\
 		.multiply = _name ## _multiply,				\
 	}