Add a target gitignore to topdir.mk, which blasts jw-pkg's default idea of
how .gitignore should look like into $(TOPDIR)/.gitignore. It's not a
prerequisite of any other target, because it's not meant to run
automatically. The idea is to keep some central maintenance of what should
be in there from jw-pkg's perspective, make it convenvient to update, but
leave the final decision to the commiter of any project using the
machinery.
Signed-off-by: Jan Lindemann <jan@janware.com>
