#ifdef reduction infrastructure, based on an argument between Shaun Jackman,

Rob Landley, and others.

Currently CONFIG options are defined or undefined, so we chop out code with
#ifdefs, ala:
#ifdef CONFIG_THING
  stuff();
#endif

This creates a new header file, bb_config.h, which sets the CONFIG entry to 1
or 0, and lets us do:

  if(CONFIG_THING) stuff();

And let the compiler do dead code elimination to get rid of it.  (Note: #ifdef
will still work because for the 1 case it's a static const int, not a #define.)

include/busybox.h

@@ -24,7 +24,7 @@
 #ifndef	_BB_INTERNAL_H_
 #define	_BB_INTERNAL_H_    1
 
-#include "config.h"
+#include "bb_config.h"
 
 #include <stdio.h>
 #include <stdlib.h>