bitbake.conf/native/nativesdk: Set PKG_CONFIG_SYSTEM_ at top level
Setting PKG_CONFIG_SYSTEM_{HEADERS/INCLUDE}_PATH for nativesdk isn't
enough, we also need to deal with multlib cases where libdir from
pkg-config-native isn't correct. Native builds are about the only
case where this variable shouldn't be set.
Therefore move the code from nativesdk to bitbake.conf and unexport it
in the native case.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
