Enable to generate directive files

To upload files on the GNU FTP, we need to sign them but also to
generate directive files[1].

[1]https://www.gnu.org/prep/maintain/maintain.html#FTP-Upload-Release-File-Triplet

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>

Makefile

@@ -49,6 +49,13 @@ all: \
 	$(DESTDIR)/acpica-unix2-20211217.tar.gz \
 	$(DESTDIR)/acpica-unix2-20220331.tar.gz \
 
+%.directive: %
+	@echo -n "" > $@
+	@echo "version: $(basename \
+		$(basename $(subst $(DESTDIR)/acpica-unix2-,,$^)))" >> $@
+	@echo "filename: $(subst $(DESTDIR)/,,$^)" >> $@
+	@echo "directory: gnuboot/mirrors/acpica.org" >> $@
+
 # 0bb0984724 gnu: acpica: Update to 20220331.
 $(DESTDIR)/acpica-unix2-20220331.tar.gz:
 	install -m 644 \