Introduce Customization struct (#458)

* Introduce Customization trait

* Introduce Customization trait including the customization accessors
  that control various behaviors.

* Expose Customization through a getter API in Env, and make the code
  that directly access the constants currently switch to accessing the
  customizations via Env.

* TockEnv's customization getter implementation directly returns the
  reference of the global DEFAULT_CUSTOMIZATION constant, so the
  constant values are still inlined and dead code won't be compiled.

* We'll add the customizations from global constants to the struct
  one-by-one, only MAX_MSG_SIZE in this commit.

* Small fixes

* Fix deploy script
* put is_valid under std gate

src/api/customization.rs

@@ -0,0 +1,62 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! This file contains all customizable constants.
+//!
+//! If you adapt them, make sure to run the tests before flashing the firmware.
+//! Our deploy script enforces the invariants.
+
+pub trait Customization {
+    /// Maximum message size send for CTAP commands.
+    ///
+    /// The maximum value is 7609, as HID packets can not encode longer messages.
+    /// 1024 is the default mentioned in the authenticatorLargeBlobs commands.
+    /// Larger values are preferred, as that allows more parameters in commands.
+    /// If long commands are too unreliable on your hardware, consider decreasing
+    /// this value.
+    fn max_msg_size(&self) -> usize;
+}
+
+#[derive(Clone)]
+pub struct CustomizationImpl {
+    pub max_msg_size: usize,
+}
+
+pub const DEFAULT_CUSTOMIZATION: CustomizationImpl = CustomizationImpl { max_msg_size: 7609 };
+
+impl Customization for CustomizationImpl {
+    fn max_msg_size(&self) -> usize {
+        self.max_msg_size
+    }
+}
+
+#[cfg(feature = "std")]
+pub fn is_valid(customization: &impl Customization) -> bool {
+    // Max message size must be between 1024 and 7609.
+    if customization.max_msg_size() < 1024 || customization.max_msg_size() > 7609 {
+        return false;
+    }
+    true
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    #[allow(clippy::assertions_on_constants)]
+    fn test_invariants() {
+        assert!(is_valid(&DEFAULT_CUSTOMIZATION));
+    }
+}