[BusyBox] autoconf stuff

[Larry Doolittle]

Erik -

> However there are some platfom specific things in
> busybox which really should be hidden when not relevant.  For example, the
> pivot_root applet isn't going to be much use on Solaris or on the HURD.

Yet.

>  So it
> makes sense to only show that option on Linux, using something like
> 
>     if [ "$LINUX" = "y" ] ; then
> 	    bool 'pivot_root'	BB_PIVOT_ROOT
>     fi

What's wrong with

     if [ "$HAS_PIVOT_ROOT" = "y" ] ; then
 	    bool 'pivot_root'	BB_PIVOT_ROOT
     fi

where $HAS_PIVOT_ROOT is determined by our mythical autoconf
replacement?  Plus, I would rather see the menu item grayed
out, than have it totally missing.

        - Larry