As wifidog-ng builds a kernel module, it must use a PKG_BUILD_DIR in
KERNEL_BUILD_DIR instead of BUILD_DIR, otherwise old build artifacts may
be incorrectly reused when switching between different targets of same
architecture without a full clean.

Instead of fixing up the override, just remove it and instead move the
kernel.mk include above package.mk, so PKG_BUILD_DIR is set up correctly
by default.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>