The host versions shouldn't be visible in Kconfig, so remove the
reference to BR2_PACKAGE_PKGCONFIG everywhere and prefix the host targets
with host-.
At the same time add pkgconfig for the target (E.G. for development) and
let BR2_PACKAGE_PKGCONFIG control that package.
Notice: all defconfigs in the tree have been updated, but make sure to
disable the pkgconfig package (unless you want it) if you use an external
config, otherwise you'll end up with pkgconfig and glib2 in the target.
PKG_CONFIG_SYSROOT_DIR and friends get set in package/Makefile.in,
so there's no need to set it in the makefiles of the individual
packages as well.
Based on a patch by Markus Heidelberg.
