[U-Boot] Broken target

The 'tools-only' target seems to be broken.

In a clean tree (no previous build) running:

$: make tools-only

fails with:

CHK include/config/uboot.release /bin/sh: 1: cannot open include/config/auto.conf: No such file Makefile:1049: recipe for target 'include/config/uboot.release' failed make: *** [include/config/uboot.release] Error 2

Any idea about the root cause? it is a regression as 2014.07 works fine in this case.