Kleaf: Get kernel release from kernel_build

Before this was a string attribute, which was error prone
because it was a fixed string, while it could change given
different configs (e.g. --config=stamp, --config=release).

After this change, the version is now inferred from the
kernel_build attribute.

The sample is also updated to showcase how to use it.

Tested: tools/bazel build //build/kernel/kleaf/tests/ddk_examples/ddk_prebuilt_module:ddk_prebuilt_module_sample_initramfs
Bug: 384949550
Change-Id: I7d8636c5956c99a20948da2243ebc94030c13144