kernel-headers: adjust PKG_ variables when using git clone method

kernel-headers: adjust PKG_ variables when using git clone method

When using an external git clone for the kernel repo,
the build would fail because the build won't download
[via git] the kernel tarball.

This is because the `toolchain/kernel-headers` assumes
that the kernel would get downloaded via normal HTTP.
The reason for this is the `HostBuild` rule, which
calls the `Download/default` rule.

To use the `Download/default` we just need to conditionally
adjust some PKG_ vars.

We can safely use `LINUX_VERSION` as it was already adjusted
in the `kernel-version.mk` to avoid collisions with other tarballs.

Fixes:
 https://bugs.openwrt.org/index.php?do=details&task_id=503

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>