[process] Pass containing object pointer to process step() methods

Give the step() method a pointer to the containing object, rather than
a pointer to the process.  This is consistent with the operation of
interface methods, and allows a single function to serve as both an
interface method and a process step() method.

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

src/net/retry.c

@@ -198,7 +198,4 @@ static void retry_step ( struct process *process __unused ) {
 }
 
 /** Retry timer process */
-struct process retry_process __permanent_process = {
-	.list = LIST_HEAD_INIT ( retry_process.list ),
-	.step = retry_step,
-};
+PERMANENT_PROCESS ( retry_process, retry_step );