linux/linux.mk: use HOST_MAKE_ENV rather than TARGET_MAKE_ENV

linux/linux.mk: use HOST_MAKE_ENV rather than TARGET_MAKE_ENV

While the kernel is built for the target, the build may need various host
libraries depending on config (and kernel version), so use HOST_MAKE_ENV
instead of TARGET_MAKE_ENV.

In particular, this ensures that our host-pkgconf will look for host
libraries and not target ones.

Fixes building scripts/dtc for Buildroot configurations enabling libyaml and
host-pkgconf for kernels after commit 067c650c45 (dtc: Use pkg-config to
locate libyaml).

With this enabled, we can drop the PKG_CONFIG_* variables for the
_NEEDS_HOST_LIBELF conditional, as those are included in HOST_MAKE_ENV.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>