Only Nano S+ is currently supported

[yogh333]

NanoX is not supported (see issue mobilecoinofficial#27), so it is useless to build it.

Moreover, when building the fw for Nano X in ledger-app-builder, through cargo ledger build nanox and using NanoX custom target file (see target NanoX defined here) which is available in the container, there is an issue with heapless crate build:

error[E0599]: no method named `compare_exchange_weak` found for reference `&AtomicU8` in the current scope
   --> /opt/.cargo/registry/src/index.crates.io-6f17d22bba15001f/heapless-0.7.16/src/mpmc.rs:226:18
    |
225 |               if dequeue_pos
    |  ________________-
226 | |                 .compare_exchange_weak(
    | |                 -^^^^^^^^^^^^^^^^^^^^^ method not found in `&AtomicU8`
    | |_________________|
    |

error[E0599]: no method named `compare_exchange_weak` found for reference `&AtomicU8` in the current scope
   --> /opt/.cargo/registry/src/index.crates.io-6f17d22bba15001f/heapless-0.7.16/src/mpmc.rs:266:18
    |
265 |               if enqueue_pos
    |  ________________-
266 | |                 .compare_exchange_weak(
    | |                 -^^^^^^^^^^^^^^^^^^^^^ method not found in `&AtomicU8`
    | |_________________|
    |

error[E0599]: no method named `fetch_add` found for struct `AtomicUsize` in the current scope
   --> /opt/.cargo/registry/src/index.crates.io-6f17d22bba15001f/heapless-0.7.16/src/pool/singleton/arc.rs:259:44
    |
259 |         let old_size = self.inner().strong.fetch_add(1, Ordering::Relaxed);
    |                                            ^^^^^^^^^ method not found in `AtomicUsize`

error[E0599]: no method named `fetch_sub` found for struct `AtomicUsize` in the current scope
   --> /opt/.cargo/registry/src/index.crates.io-6f17d22bba15001f/heapless-0.7.16/src/pool/singleton/arc.rs:307:32
    |
307 |         if self.inner().strong.fetch_sub(1, Ordering::Release) != 1 {
    |                                ^^^^^^^^^ method not found in `AtomicUsize`

error[E0599]: no method named `compare_exchange_weak` found for struct `AtomicPtr` in the current scope
  --> /opt/.cargo/registry/src/index.crates.io-6f17d22bba15001f/heapless-0.7.16/src/pool/llsc.rs:43:29
   |
43 |             match self.head.compare_exchange_weak(
   |                   ----------^^^^^^^^^^^^^^^^^^^^^ method not found in `AtomicPtr<Node<T>>`

error[E0599]: no method named `compare_exchange_weak` found for struct `AtomicPtr` in the current scope
  --> /opt/.cargo/registry/src/index.crates.io-6f17d22bba15001f/heapless-0.7.16/src/pool/llsc.rs:64:33
   |
64 |                 match self.head.compare_exchange_weak(
   |                       ----------^^^^^^^^^^^^^^^^^^^^^ method not found in `AtomicPtr<Node<T>>`

   Compiling hmac v0.12.1
For more information about this error, try `rustc --explain E0599`.
error: could not compile `heapless` (lib) due to 6 previous errors

This issue may be due to https://doc.rust-lang.org/std/sync/atomic/index.html#portability.
The issue is not seen when building with the target file provided in the repo (i.e. fw/nanox.json) with cargo build --targe ./nanox.json --release

When comparing both target files (Ledger vs Mobilecoin), there are significant differences, for instance llvm-target.

As nowadays NanoX is not supported, I propose to build only for Nano S+ device.