jool: fix PKG_BUILD_DIR to avoid kernel ABI mismatch

jool: fix PKG_BUILD_DIR to avoid kernel ABI mismatch

As jool builds a kernel module, a PKG_BUILD_DIR under KERNEL_BUILD_DIR must
be used to avoid reusing build artifacts when switching to a different
target of the same architecture. Otherwise, kernel ABI mismatches may
result, leading to an unusuable module, or build failures like the
following:

    Package kmod-jool is missing dependencies for the following libraries:
    crypto_hash.ko

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