Fix kernel hashes build failure

[CookieComputing]

When I was trying to build stage0 with the new sev_kernel_hashes feature introduced in 6fe1a11, I noticed that I was getting build failures:

ERROR: /home/kevinhui/oak/stage0/BUILD:25:13: Compiling Rust rlib stage0 (37 files) failed: (Exit 1): process_wrapper failed: error executing Rustc command (from target //stage0:stage0) bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust+/util/process_wrapper/process_wrapper --arg-file ... (remaining 168 arguments skipped)

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
error[E0599]: no method named `as_bytes` found for struct `KernelCmdLine` in the current scope
   --> stage0/src/lib.rs:223:21
    |
223 |             cmdline.as_bytes(),
    |                     ^^^^^^^^ method not found in `KernelCmdLine`
    |
   ::: stage0/src/kernel.rs:120:1
    |
120 | pub struct KernelCmdLine {
    | ------------------------ method `as_bytes` not found for this struct
    |
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following trait defines an item `as_bytes`, perhaps you need to implement it:
            candidate #1: `IntoBytes`

error: aborting due to 1 previous error

This is replicable with the following changes:

[kevinhui ~/oak (main)]$ git diff
diff --git a/stage0/BUILD b/stage0/BUILD
index ddf5dd06d1..d08bc2a2e1 100644
--- a/stage0/BUILD
+++ b/stage0/BUILD
@@ -26,7 +26,7 @@ rust_library(
     name = "stage0",
     srcs = glob(["src/**"]),
     # Enable if you want to use kernel hashes
-    # crate_features = ["sev_kernel_hashes"],
+    crate_features = ["sev_kernel_hashes"],
     target_compatible_with = any_platform([
         "//:x86_64-none-setting",
         "//:x86_64-none-no_avx-setting",
diff --git a/stage0_sev/BUILD b/stage0_sev/BUILD
index 169115a6f7..d923906883 100644
--- a/stage0_sev/BUILD
+++ b/stage0_sev/BUILD
@@ -25,7 +25,7 @@ rust_library(
     name = "stage0_sev",
     srcs = glob(["src/**/*.rs"]),
     # Enable if you want to use kernel hashes
-    # crate_features = ["sev_kernel_hashes"],
+    crate_features = ["sev_kernel_hashes"],
     aliases = {"//stage0": "oak_stage0"},
     deps = [
         "//oak_attestation",

After this PR, those build failures go away:

[kevinhui ~/oak (fix-kernel-hashes-build)]$ bazel build //stage0_bin
INFO: Analyzed target //stage0_bin:stage0_bin (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //stage0_bin:stage0_bin up-to-date:
  bazel-bin/stage0_bin/stage0_bin
INFO: Elapsed time: 8.894s, Critical Path: 8.34s
INFO: 4 processes: 1 internal, 3 linux-sandbox.
INFO: Build completed successfully, 4 total actions

I also took the liberty to add a note to stage0_bin/README to add a comment about kernel hashes

[conradgrobler]

Thanks! Could you please run just format? The linter is failing on the README change.

[CookieComputing]

Thanks! Just formatted it

[conradgrobler]

Imported as d1fb4aa