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.
libpng used to have the 'png' Makefile alias, which some packages used
in their dependencies list.
With the move to Makefile.autotools.in this is now gone, so update the
packages to match.
