Download of Source takes a long time

The `build.sh` clones the entire repo and then immediately switches to a single branch:
https://github.com/intel/linux-kernel-overlay/blob/02bedab64a4a4e80a524015ab02bf252f2c1068e/build.sh#L57C2-L60

This could be streamlined by downloading a single branch, with something like:

``` shell
git clone $KSRC_REPO --branch $KSRC_UPSTREAM_TAG --single-branch $BUILD_DIR
```

and remove the push / pop.

This downloads a smaller subset and is much faster.