Use LedgerHQ#38 in build.rs and structure

The first change is to parse once, and then use an enum for the device.
This is hopefully a straightforward improvement.

The second change is to case on the OS name (leveraging LedgerHQ#38) instead of
the target name. The target "name" isn't so structured, and it is
unclear to what extent it should be anm exposed part of the target. (See
rust-lang/rust#98225 for example, where the
contents rather than json file path were used as a a key.)

With LedgerHQ#38 the device name is used for the OS field instead, and so we are
robust to confusing behavior around names.

build.rs

@@ -182,18 +182,30 @@ fn main() -> Result<(), Box<dyn Error>> {
         .flag("-Wno-unused-command-line-argument")
         .clone();
 
-    // determine target
-    let target = env::var_os("TARGET");
-    let cx_makefile = match target.clone().unwrap().to_str().unwrap() {
-        "nanos" => finalize_nanos_configuration(&mut command, &bolos_sdk),
-        "nanox" => finalize_nanox_configuration(&mut command, &bolos_sdk),
-        "nanosplus" => finalize_nanosplus_configuration(&mut command, &bolos_sdk),
+    enum Device {
+        NanoS,
+        NanoSPlus,
+        NanoX,
+    }
+    use Device::*;
+
+    // determine device
+    let device = match env::var_os("CARGO_CFG_TARGET_OS").unwrap().to_str().unwrap() {
+        "nanos" => NanoS,
+        "nanosplus" => NanoSPlus,
+        "nanox" => NanoX,
         target_name => panic!(
             "invalid target `{}`, expected one of `nanos`, `nanox`, `nanosplus`. Run with `-Z build-std=core --target=./<target name>.json`",
             target_name
         ),
     };
 
+    let cx_makefile = match NanoS {
+        NanoS => finalize_nanos_configuration(&mut command, &bolos_sdk),
+        NanoX => finalize_nanox_configuration(&mut command, &bolos_sdk),
+        NanoSPlus => finalize_nanosplus_configuration(&mut command, &bolos_sdk),
+    };
+
     // all 'finalize_...' functions also declare a new 'cfg' variable corresponding
     // to the name of the target (as #[cfg(target = "nanox")] does not work, for example)
     // this allows code to easily import things depending on the target
@@ -225,11 +237,10 @@ fn main() -> Result<(), Box<dyn Error>> {
     // extend the library search path
     println!("cargo:rustc-link-search={}", out_dir.display());
     // copy
-    let linkerscript = match target.unwrap().to_str().unwrap() {
-        "nanos" => "nanos_layout.ld",
-        "nanox" => "nanox_layout.ld",
-        "nanosplus" => "nanosplus_layout.ld",
-        _ => "",
+    let linkerscript = match device {
+        NanoS => "nanos_layout.ld",
+        NanoX => "nanox_layout.ld",
+        NanoSPlus => "nanosplus_layout.ld",
     };
     std::fs::copy(linkerscript, out_dir.join(linkerscript))?;
     std::fs::copy("link.ld", out_dir.join("link.ld"))?;