kbuild: override build timestamp & version

Introduce KBUILD_BUILD_VERSION to make it
possible to override kernel build version
during build time.

Introduce KBUILD_BUILD_TIMESTAMP to make it
possible to override kernel build timestamp
during build time.

But variables are useful mainly by distros
that want to pass info from an SCM when
building the kernel. Timestamp could be last
checkin date for a file etc.

The idea came from Olaf Hering <olaf@aepfle.de>

Cc: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>