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>

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 = (