build: inform users of deprecation

Print a warning in case users attempt to build Gluon using a site.mk
which containes deprecated GLUON_FEATURES or GLUON_SITE_PACKAGES.

Signed-off-by: David Bauer <mail@david-bauer.net>

Makefile

@@ -35,6 +35,14 @@ ifneq ($(GLUON_BRANCH),)
   GLUON_AUTOUPDATER_ENABLED ?= 1
 endif
 
+ifneq ($(GLUON_FEATURES)$(GLUON_FEATURES_standard)$(GLUON_FEATURES_tiny),)
+  $(error *** Warning: GLUON_FEATURES has been obsolete, please use the image-customization file instead.)
+endif
+
+ifneq ($(GLUON_SITE_PACKAGES)$(GLUON_SITE_PACKAGES_standard)$(GLUON_SITE_PACKAGES_tiny),)
+  $(error *** Warning: GLUON_SITE_PACKAGES has been obsolete, please use the image-customization file instead.)
+endif
+
 GLUON_AUTOUPDATER_ENABLED ?= 0
 
 # initialize (possibly already user set) directory variables