linux: use host pkg-config when host libelf is set

A patch was added to the Linux kernel in 5.1.0-rc3 which adds a
requirement that the host build environment include pkg-config. Add the
correct host-pkgconf dependency and environment variables to ensure
Linux picks up the correct libraries.

Move the existing LINUX_MAKE_ENV assignment earlier, to simplify the
append-assignment in the libelf conditional block.

Fixes: #11761

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Arnout: extend commit message as suggested by Yann]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>