build: warn when packages have no associated install section

Declaring a package without the appropriate install section is an easy
mistake to make, especially when renaming packages. Since this is also
easy to detect, warn about it when it happens.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
No-objections-at-all-by: Jo-Philipp Wich <jo@mein.io>

include/package-ipkg.mk

@@ -124,6 +124,8 @@ ifeq ($(DUMP),)
       endif
       $(PKG_INSTALL_STAMP).$(1): prepare-package-install
 		echo "$(1)" >> $(PKG_INSTALL_STAMP)
+    else
+      $(if $(CONFIG_PACKAGE_$(1)),$$(warning WARNING: skipping $(1) -- package has no install section))
     endif
     endif