message: Remove support for `platform` file.
  The support for the `platform` file broke when `libtock_platform`
  was moved into the `platform` directory. If not specifying the
  `PLATFORM` environment variable, the build currently breaks, so
  it appears that the `platform` file support us not being usued.
diff: use std::env;
  use std::fs;
  -use std::fs::File;
  -use std::io::BufRead;
  -use std::io::BufReader;
  use std::path::{Path, PathBuf};
  use std::process;
  @@ -11,19 +8,16 @@ static LAYOUT_GENERIC_FILENAME: &str = "layout_generic.ld";
  fn main() {
  static PLATFORM_ENV_VAR: &str = "PLATFORM";
  - static PLATFORM_FILE_NAME: &str = "platform";
  static APP_HEAP_SIZE: &str = "APP_HEAP_SIZE";
  static KERNEL_HEAP_SIZE: &str = "KERNEL_HEAP_SIZE";
  println!("cargo:rerun-if-env-changed={}", PLATFORM_ENV_VAR);
  println!("cargo:rerun-if-env-changed={}", APP_HEAP_SIZE);
  println!("cargo:rerun-if-env-changed={}", KERNEL_HEAP_SIZE);
  - println!("cargo:rerun-if-changed={}", PLATFORM_FILE_NAME);
  println!("cargo:rerun-if-changed={}", LAYOUT_FILE_NAME);
  println!("cargo:rerun-if-changed={}", LAYOUT_GENERIC_FILENAME);
  - let platform_name =
  - read_env_var(PLATFORM_ENV_VAR).or_else(|| read_board_name_from_file(PLATFORM_FILE_NAME));
  + let platform_name = read_env_var(PLATFORM_ENV_VAR);
  if let Some(platform_name) = platform_name {
  println!("cargo:rustc-env={}={}", PLATFORM_ENV_VAR, platform_name);
  copy_linker_file(platform_name.trim());
  @@ -51,20 +45,6 @@ fn read_env_var(env_var: &str) -> Option<String> {
  env::var_os(env_var).map(|os_string| os_string.into_string().unwrap())
  }
  -fn read_board_name_from_file(file_name: &str) -> Option<String> {
  - let path = Path::new(file_name);
  - if !path.exists() {
  - return None;
  - }
  -
  - let board_file = File::open(path).unwrap();
  - let mut board_name = String::new();
  - BufReader::new(board_file)
  - .read_line(&mut board_name)
  - .unwrap();
  - Some(board_name)
  -}
  -
  fn copy_linker_file(platform_name: &str) {
  let linker_file_name = format!("boards/layout_{}.ld", platform_name);
  let path = Path::new(&linker_file_name);
  