core/pkg-kconfig: fix configurators

Currently, the configurators are using $($(2)_MAKE_ENV), often derived
from $(TARGE_MAKE_ENV), as the environment to be set when calling the
various configurators.

This means that our host tools are used first, most notably pkg-config
(from host-pkgconf).

However, this is inherently flawed. Our pkg-config, when set for the
host, only searches .pc files in $(HOST_DIR) and never ever uses the
ones from the host. For example, since we do not build a host-qt, our
pkg-config would not find the host's QtCore.pc et al.

Consequently, on some systems (but not on others?) most of the
configurators fail to build, especially the latest kernel versions, as
they have been starting to use pkg-config two years ago.

Fix that by filtering-out sensible values out of the environment, but
only when calling the configurators.

[Thomas: rewrap comment to appropriate length.]

Reported-by: Mauro Condarelli <mc5686@mclink.it>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Mauro Condarelli <mc5686@mclink.it>
Tested-by: Mauro Condarelli <mc5686@mclink.it>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>