luci-base: Extending packages prepare handling

With this change, packages that do not have the source files in the
generic location (luasrc,ucode,htdocs,root,src) can still copy there
build files into the build directory if they add the following make
target 'Build/Prepare/<luci-packages-name>' to the Makefile.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

luci.mk

@@ -174,6 +174,7 @@ define Build/Prepare
 		$(CP) ./$$$$d/* $(PKG_BUILD_DIR)/$$$$d/; \
 	  fi; \
 	done
+	$(call Build/Prepare/$(LUCI_NAME))
 	$(call Build/Prepare/Default)
 endef