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 = (