invoke from kernel makefile

[chrislusf]

Using branch 5.11, and cd bento/rust folder, cargo build suggested invoke from kernel makefile. Not very clear how to do that.

chrislu@chrislu-VirtualBox:~/dev/bento/bento/rust$ cargo build
   Compiling bento v0.1.0 (/home/chrislu/dev/bento/bento/rust)
error: failed to run custom build command for `bento v0.1.0 (/home/chrislu/dev/bento/bento/rust)`

Caused by:
  process didn't exit successfully: `/home/chrislu/dev/bento/bento/rust/target/debug/build/bento-117c02fd10b39823/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-env-changed=CC
  cargo:rerun-if-env-changed=KDIR
  cargo:rerun-if-env-changed=c_flags

  --- stderr
  thread 'main' panicked at 'Must be invoked from kernel makefile: NotPresent', build.rs:387:39
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[smiller123]

Run make from the rust directory under the file system you want to compile. The code in the bento directory will be compiled as part of compiling the file system that uses the bento crate.