Split the (quick hack) boot logic out from main.c to autoboot.c, add a

"boot" command to attempt booting from within the command shell, fall back
to shell if boot fails for any reason.

src/include/gpxe/autoboot.h

@@ -0,0 +1,12 @@
+#ifndef _GPXE_AUTOBOOT_H
+#define _GPXE_AUTOBOOT_H
+
+/** @file
+ *
+ * Automatic booting
+ *
+ */
+
+extern void autoboot ( void );
+
+#endif /* _GPXE_AUTOBOOT_H */