rules.mk: make PKG_CONFIG_DEPENDS properly track string values

This will ensure that gluon-site and gluon-autoupdater are rebuilt
correctly when GLUON_RELEASE or GLUON_BRANCH change.

patches/lede/0010-rules.mk-make-PKG_CONFIG_DEPENDS-properly-track-string-values.patch

+From: Matthias Schiffer <mschiffer@universe-factory.net>
+Date: Fri, 24 Feb 2017 12:16:33 +0100
+Subject: rules.mk: make PKG_CONFIG_DEPENDS properly track string values
+
+The confvar macro is adjusted to not only consider if a variable has a
+value or not, but also the value itself. Instead of creating a string of
+'y' and 'n' characters, all variable names and values are concatenated
+and hashed.
+
+Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
+
+diff --git a/rules.mk b/rules.mk
+index 1b9de5f35e3890c2759ab2cdbd98ac80df942124..626bfca1ba73b96ee0f86a2de4afe643d83b360c 100644
+--- a/rules.mk
++++ b/rules.mk
+@@ -29,7 +29,7 @@ empty:=
+ space:= $(empty) $(empty)
+ comma:=,
+ merge=$(subst $(space),,$(1))
+-confvar=$(call merge,$(foreach v,$(1),$(if $($(v)),y,n)))
++confvar=$(shell echo '$(foreach v,$(1),$(v)=$(subst ','\'',$($(v))))' | $(STAGING_DIR_HOST)/bin/mkhash md5)
+ strip_last=$(patsubst %.$(lastword $(subst .,$(space),$(1))),%,$(1))
+ 
+ paren_left = (