scanner-support: add a package for the scanner group

Scanner access is granted per device: a USB scanner is matched by vendor
and product id against a database of supported devices, which only the
driver package has. There is no device class to write a generic hotplug
rule against, and putting every USB device node into a group would grant
far more than scanner access.

Allocate the scanner group and nothing else, so the id is the same on
every image, one package owns it, and scanner userspace has something to
depend on. The rule granting access to a particular device stays with
the driver package.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/system/hardware-support/Makefile

@@ -141,6 +141,24 @@ define Package/printer-support/install
 	$(call InstallFragment,$(1),55-printer.json)
 endef
 
+define Package/scanner-support
+  $(call Package/hardware-support/Default)
+  TITLE:=Scanner device support
+  USERID:=:scanner=47
+endef
+
+define Package/scanner-support/description
+ Feature gate for scanners. Allocates the scanner group, so that the id
+ is the same on every image and one package owns it. Ships no hotplug
+ rule: a USB scanner is matched by vendor and product id against a
+ database of supported devices, which only the driver package has, so
+ the rule granting access to a particular scanner stays there.
+endef
+
+define Package/scanner-support/install
+	$(INSTALL_DIR) $(1)
+endef
+
 define Build/Prepare
 endef
 
@@ -157,3 +175,4 @@ $(eval $(call BuildPackage,input-support))
 $(eval $(call BuildPackage,dvb-support))
 $(eval $(call BuildPackage,kvm-support))
 $(eval $(call BuildPackage,printer-support))
+$(eval $(call BuildPackage,scanner-support))