linux: override build timestamp for reproducible builds

linux: override build timestamp for reproducible builds

Linux kernel include a few information about build environment in its binary.
This feature is incompatible with BR2_REPRODUCIBLE. This patch overload build
information when BR2_REPRODUCIBLE is enabled.

Note that usage of KBUILD_BUILD_TIMESTAMP is not mandatory since Buildroot
use `fakedate'. However, native solution is prefered when upstream
provide one.

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>