Makefile: Fix README.html export.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Use weather-metno-pkg.el.in.

This should allow MELPA to extract dependencies and other package
information while not duplicating the effort in `weather-metno.el'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>