[interface] Allow for the definition of an unused interface operation

Allow an interface operation to be declared as unused.  This will
perform full type-checking and compilation of the implementing method,
without including any code in the resulting object (other than a NULL
entry in the interface operations table).

The intention is to provide a relatively clean way for interface
operation methods to be omitted in builds for which the operation is
not required (such as an operation to describe an object using an EFI
device path, which would not be required in a non-EFI build).

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

src/include/ipxe/interface.h

@@ -36,6 +36,21 @@ struct interface_operation {
 			  ? op_func : op_func ),			      \
 	}
 
+/**
+ * Define an unused object interface operation
+ *
+ * @v op_type		Operation type
+ * @v object_type	Implementing method's expected object type
+ * @v op_func		Implementing method
+ * @ret op		Object interface operation
+ */
+#define UNUSED_INTF_OP( op_type, object_type, op_func ) {		      \
+		.type = NULL,						      \
+		.func = ( ( ( ( typeof ( op_func ) * ) NULL ) ==	      \
+			    ( ( op_type ## _TYPE ( object_type ) * ) NULL ) ) \
+			  ? NULL : NULL ),				      \
+	}
+
 /** An object interface descriptor */
 struct interface_descriptor {
 	/** Offset of interface within containing object */