default: use clone-depth=1 for kernel projects

The am62x-kernel.git project uses over 8GiB of storage. Since we don't need the git history of kernel binaries, do a shallow clone instead.

This reduces the overall storage usage of our android repo and is a common practice for binary projects in repo.

Signed-off-by: Mattijs Korpershoek mkorpershoek@baylibre.com