Globally disable PKG_CONFIG_PATH

If PKG_CONFIG_PATH is set in the environment, it allows pkg-config to
look for libraries outside of the buildroot tree.  This is a problem
both for host and target builds.  To avoid it, globally set an empty
PKG_CONFIG_PATH in the environment.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>