uboot-tools: pass empty CFLAGS on invocation of libfdt/setup.py

uboot-tools: 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"

Fixes:
  http://autobuild.buildroot.net/results/439d6dacfaa7f9244bd15ca04990c12a25ff4777
  http://autobuild.buildroot.net/results/e2fe36c20e75d01a01a6659ef11e888965a843b9
  http://autobuild.buildroot.net/results/6f37ff347822a76571a9fe9cf3c491fe7e4164fa

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>