Header rearrangement.

I want to get to the point where any header in include/ reflects a
standard user-level header (e.g. a POSIX header), while everything that's
specific to gPXE lives in include/gpxe/.  Headers that reflect a Linux
header (e.g. if_ether.h) should also be in include/gpxe/, with the same
name as the Linux header and, preferably, the same names used for the
definitions.

src/include/image.h

@@ -3,7 +3,7 @@
 
 #include "stdint.h"
 #include "io.h"
-#include "tables.h"
+#include <gpxe/tables.h>
 #include "dev.h"
 
 struct image {