Makefile: compute the version string for all to use

Some packages use the buildroot version string (to set their own version
string). Computing the version string globally will makes it easy for
them to use it instead of re-computing it in every packages.

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