Add NULL_TRAP as a config option

src/config.h

 				 * "make bin/rtl8139.dsk bs" */
 #undef	BUILD_ID		/* Include a custom build ID string,
 				 * e.g "test-foo" */
+#undef	NULL_TRAP		/* Attempt to catch NULL function calls */
 
 /* @END general.h */

src/core/config.c

 #ifdef	PXE_IMAGE
 REQUIRE_OBJECT ( pxe );
 #endif
+
+/*
+ * Drag in miscellaneous objects
+ *
+ */
+#ifdef	NULL_TRAP
+REQUIRE_OBJECT ( nulltrap );
+#endif