core/pkg-kconfig: pass host PKG_CONFIG_PATH env var

This is basically the same change as in 0515fe45
("Makefile: pass host PKG_CONFIG_PATH at "make menuconfig" time"). That
commit made sure to pass host PKG_CONFIG_PATH when invoking Buildroot's
own menuconfig program. This change ensures that the same is true for
third party menuconfig programs (i.e. Linux, uClibc and Busybox).

This unbreaks "make {linux,uclibc}-menuconfig" for host platforms which
rely on PKG_CONFIG_PATH to find .pc files (e.g. NixOS). (When Busybox
updates to a more recent Kconfig snapshot, one that uses pkg-config to
find ncurses, "make busybox-menuconfig" will also start working.)

Tested on Ubuntu and NixOS:

  $ make qemu_arm_versatile_defconfig
  $ make linux-menuconfig
  $ make

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>