[U-Boot] tools-only build broken

[Ian Campbell]

The tools-only target is currently broken:
        $ make CROSS_COMPILE=arm-linux-gnueabihf- tools-only 
          CHK     include/config/uboot.release
        /bin/sh: 1: cannot open include/config/auto.conf: No such file
        Makefile:1052: recipe for target 'include/config/uboot.release' failed
        make: *** [include/config/uboot.release] Error 2
        
This is because tools-only depends on $(version_h), depends on
include/config/uboot.release, depends include/config/auto.conf. I think
this is all due to the support for LOCALVERSION(_AUTO).

I'm not sure how to reconcile this, since the version stuff
fundamentally needs the .config. Perhaps sandbox could be used?

Ian.