Makefile: move version string from Config.in into Makefile

This re-instates writing the version string in .config headers, and no
longer provides it as a kconfig symbol in .config (it is now a variable
in the Makefile, and in the environment).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>