feat: add DTS include support for build-time device tree generation

[xuchang-vivo]

Summary

Integrate the DTS build framework from the build repository into the kernel.

• BUILD.gn: Added dts_gen dependency (dts_gen_${board}) to both blueos and kernel_unittest targets. Added DTS_OUT_DIR rustenv variable so the generated module path is resolved at compile time via an environment variable instead of a hardcoded path.
• mod.rs: Added include!(concat!(env!("DTS_OUT_DIR"), "/mod.rs")) to pull in build-time generated device tree constants.

Architecture

The DTS_OUT_DIR env var is set in GN via rustenv and points to root_gen_dir/dts (rebased to absolute path). This removes the hardcoded path dependency and makes the include portable across different build environments.

Test Plan

• ninja -C out/qemu_riscv64 check_all — all 139 targets pass
• Unit tests and integration tests pass on qemu_riscv64

Co-authored-by: Claude Haiku 4.5 noreply@anthropic.com