Remove .gitkeep, adapt Makefile

The .gitkeep file was getting into the debian package causing
spurious errors during install.

debian.package/Makefile

@@ -18,9 +18,12 @@ uefi-boot-metadata-collect-agent/boot/efi/uefi-boot-metadata-collect-agent.efi:
 	sudo chmod 755 $@
 	rm -rfv /tmp/dracut-empty
 
-uefi-boot-metadata-collect-agent/boot/efi/secboot.cer: db.cer
+uefi-boot-metadata-collect-agent/boot/efi/secboot.cer: uefi-boot-metadata-collect-agent/boot/efi db.cer
 	cp -fv db.cer $@
 
+uefi-boot-metadata-collect-agent/boot/efi:
+	mkdir -m 700 $@
+
 db.crt db.key:
 	openssl req -newkey rsa:4096 -nodes -keyout db.key -new -x509 -sha256 -days 3650 -subj "/CN=Nexedi's UEFI Signature Database key/" -out db.crt