version: export tweak version in version.h

The commit export TWEAK version to code through version.h.
This aligns the symbols available in code with those available in
CMake and Kconfig.

The new define is available for both KERNEL, APP, and custom version
types (such as MCUBOOT).

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>