Bazel-ify Linux kernel build

[leoluk]

We currently build the Linux kernel in a Bazel genrule. We do not frequently change it, but when we do, it means doing a full rebuild. Long-term, we should Bazel-ify the kernel so we can get proper caching, toolchain integration and incremental rebuilds.

This is a large (but entirely plausible) task that we won't be able to get to anytime soon. It would likely be a "kazelle" approach, where we emit BUILD files for a given kconfig.