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>