build: fix host menu config targets using ncurses

On a recent Gentoo Linux installation, invoking `make menuconfig`, `make
kernel_menuconfig` or `make kernel_nconfig` in the build system fails,
whereas for example `make menuconfig` in the kernel tree alone works as
expected.

This is happening because STAGING_PREFIX is not defined when kernel's
{menu,n}config target calls pkg-config from the toolchain/host and thus
pkg-config returns an empty value, and the fallback values in the kernel
config script are applied but those are off and the linking fails.

Solution is to use system's pkg-config for all ncurses based menu config
targets in order to provide proper compiler/linker flags.

Ref: FS#2423
Cc: Thomas Albers <thomas.gameiro@gmail.com>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>
Tested-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

Makefile

@@ -18,6 +18,7 @@ $(if $(findstring $(space),$(TOPDIR)),$(error ERROR: The path to the OpenWrt dir
 
 world:
 
+DISTRO_PKG_CONFIG:=$(shell which -a pkg-config | grep -E '\/usr' | head -n 1)
 export PATH:=$(TOPDIR)/staging_dir/host/bin:$(PATH)
 
 ifneq ($(OPENWRT_BUILD),1)