add evfb (evdev + fbdev) backend

Outside of a terminal emulator or web browser, this typing experience
can be useful outside the DE, such as in the TTY or when entering a
password for full-disk encryption. Add a target that uses the bare
hardware to allow the keyboard to be deployed in more flexible
environments.

build.rs

@@ -22,4 +22,17 @@ fn main()
     println!("cargo::rustc-link-lib=expat");
 
     bindings.write_to_file(out_file).expect("Writing failure");
+
+    let builder = bindgen::builder();
+
+    let bindings = builder.header("/usr/include/linux/fb.h")
+                          .generate()
+                          .expect("The Linux headers must be installed");
+
+    let out_dir: PathBuf = env::var("OUT_DIR")
+                               .expect("Environment variable $OUT_DIR must be defined")
+                               .into();
+    let out_file = out_dir.join("linuxfb.rs");
+
+    bindings.write_to_file(out_file).expect("Writing failure");
 }