Pass empty CFLAGS on invocation of libfdt/setup.py

Pass empty CFLAGS on invocation of libfdt/setup.py

When building u-boot tools in cross-build environment CFLAGS environment
variable set up for target is taken into an account when building code
for host. Make it empty on invocation of python.

This fixes the following build errors when cross-compiling for xtensa:

  cc1: error: unrecognized command line option "-mlongcalls"
  cc1: error: unrecognized command line option "-mauto-litpools"

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>