kleaf: Move kernel_build_config to its own module.

This makes kernel.bzl smaller.

Clients are still expected to load kernel_build_config
from kernel.bzl. kernel_build_config.bzl is an implementation
detail.

Test: TH
Test: bazel build //common/... //common-modules/... --nobuild
Bug: 233156052
Change-Id: I9322a3286dc98e4313330d165ba37502d0a37eb3
diff --git a/build/kernel b/build/kernel
index 8faaa40..a809616 160000
--- a/build/kernel
+++ b/build/kernel
@@ -1 +1 @@
-Subproject commit 8faaa40a2d137c07397b197b9c186102fda1cf52
+Subproject commit a809616e3004c3b7f5b4a66fe17333598ef37ab8