Added basic code for implementing co-operative multitasking.

Yes, you really can do it in 65 bytes.

src/include/gpxe/process.h

@@ -0,0 +1,32 @@
+#ifndef _GPXE_PROCESS_H
+#define _GPXE_PROCESS_H
+
+/** @file
+ *
+ * Processes
+ *
+ */
+
+#include <gpxe/list.h>
+
+/** A process */
+struct process {
+	/** List of processes */
+	struct list_head list;
+	/**
+	 * Single-step the process
+	 *
+	 * This method should execute a single step of the process.
+	 * Returning from this method is isomorphic to yielding the
+	 * CPU to another process.
+	 *
+	 * If the process wishes to be executed again, it must re-add
+	 * itself to the run queue using schedule().
+	 */
+	void ( * step ) ( struct process *process );
+};
+
+extern void schedule ( struct process *process );
+extern void step ( void );
+
+#endif /* _GPXE_PROCESS_H */