Makefile: pass host PKG_CONFIG_PATH at "make menuconfig" time

Buildroot unexports PKG_CONFIG_PATH in the top-level Makefile for purity
reasons. But it has an unfortunate side-effect in that "make menuconfig"
will not (necessarily) be able to pick up ncurses via host pkg-config,
breaking "make menuconfig" on systems where ncurses is installed in a
non-standard location.

This patch saves the original PKG_CONFIG_PATH variable in
HOST_PKG_CONFIG_PATH and restores the original PKG_CONFIG_PATH variable
only in the sub-processes that builds the various menuconfig/nconfig/...
targets.

(PKG_CONFIG_PATH has to be placed in front of the make command so that it
propagates to sub-processes. If given as an argument, it doesn't work.)

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>