build: New script to ease packaging of gnupg

Often GnuPG is build on the same machine as GnuPG-Desktop. This
scripts makes it easy to add a new GnuPG version to the packages file.
Simply remove old gnupg versions:

cd packages
rm gnupg-*

run

./gen-gnupg.sh ~/b/gnupg/dist/gnupg-w32....tar.xz

so that the scripts copies all needed files from the gnupg build
directory to the local director. Finally edit packages.N and delete
the old version info.