Generate Genode version string on each build

The version string was generated when core/main.cc was rebuilt, which
happens on changes in the file itself or in a header file it includes.
But, the version should reflect if the Genode repository was changed at
any place. Therefore, I moved the version string to its own version.cc
which is forcedly rebuilt any time core is examined by the build system.

@nfeske what do you think about it? Core is now relinked on any build.

Fixes #551.