coreutils: disable stdbuf for static builds

When stdbuf is enabled, it builds a shared library, but it will fail
in a static-only scenario.

Fixes:

  http://autobuild.buildroot.org/results/974/9741cc9841fa95f7430a91f585c4464ee2c9311b/



Signed-off-by: David du Colombier <0intro@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>