[Settings] Expose SMBIOS via settings API

In particular, expose the system UUID as a setting ("smbios/uuid").

src/include/gpxe/uuid.h

@@ -8,9 +8,6 @@
 
 #include <stdint.h>
 
-union uuid;
-#include <bits/uuid.h>
-
 /** A universally unique ID */
 union uuid {
 	/** Canonical form (00000000-0000-0000-0000-000000000000) */