When building on x86, the final image building step always emits stats
to stderr, even though this information is neither a warning nor an error:

  BUILD   arch/x86/boot/bzImage
Setup is 16188 bytes (padded to 16384 bytes).
System is 6368 kB
CRC cbe50c61

Validating automated builds would be cleaner if stderr did not have to
filter out these lines. Instead, change how tools/build is called, and
make the zoffset header unconditional, and write to a specified file
instead of to stdout, which can then be used for statistics, leaving
stderr open for legitimate warnings and errors, like the output from
die().
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/20130906181532.GA31260@www.outflux.netSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>