swap out curve25519 backend for vexii-test targets

we don't have a curve25519 accelerator on the vexii-test device
due to a lack of space to fit it. so for vexii-test targets
we swap in the u32 (vs u32e) backend.

This requires re-writing the config.toml file based on the
presence (or absence) of a vexii-test flag.

xtask/src/main.rs

@@ -7,7 +7,7 @@ use utils::*;
 mod builder;
 use builder::*;
 mod verifier;
-use std::env;
+use std::{env, fs, path::PathBuf};
 
 use verifier::*;
 
@@ -166,6 +166,9 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     builder.add_services(&extra_services);
     // extract features, and especially track language features
     let features = get_flag("--feature")?;
+
+    setup_curve25519_backend(&features);
+
     let mut language_set = false;
     for feature in features {
         builder.add_feature(&feature);
@@ -945,3 +948,72 @@ fn get_flag(flag: &str) -> Result<Vec<String>, DynError> {
     }
     Ok(list)
 }
+
+fn setup_curve25519_backend(features: &Vec<String>) {
+    // Determine workspace root (xtask assumed to be in subdir)
+    let workspace_root = locate_workspace_root().expect("Failed to find workspace root");
+    let config_path = workspace_root.join(".cargo/config.toml");
+
+    // assume that the system has the curve25519 accelerator if it's not the vexii-test config
+    let has_accelerator = !features.contains(&"vexii-test".to_string());
+
+    // Desired cfg value
+    let desired_cfg = if has_accelerator {
+        "curve25519_dalek_backend=\"u32e_backend\""
+    } else {
+        "curve25519_dalek_backend=\"u32_backend\""
+    };
+
+    // Read original lines
+    let original = fs::read_to_string(&config_path).expect("Failed to read config.toml");
+    let mut modified = false;
+    let mut in_target = false;
+
+    let lines: Vec<String> = original
+        .lines()
+        .map(|line| {
+            let trimmed = line.trim();
+
+            // Detect section
+            if trimmed.starts_with("[target.") && trimmed.contains("riscv32imac-unknown-xous-elf") {
+                in_target = true;
+                return line.to_string();
+            }
+
+            if in_target && trimmed.starts_with('[') && trimmed != "[target.riscv32imac-unknown-xous-elf]" {
+                in_target = false; // exited the block
+            }
+
+            if in_target && trimmed.starts_with("rustflags") {
+                if trimmed.contains(desired_cfg) {
+                    return line.to_string(); // already correct
+                }
+
+                // Replace line with correct cfg
+                modified = true;
+                return format!(r#"rustflags = ["--cfg", '{}']"#, desired_cfg);
+            }
+
+            line.to_string()
+        })
+        .collect();
+
+    // Only write if modified
+    if modified {
+        fs::write(&config_path, lines.join("\n")).expect("Failed to write config.toml");
+        println!("Updated .cargo/config.toml");
+    } else {
+        println!("No change needed in .cargo/config.toml");
+    }
+}
+
+/// Traverse upward to find workspace root (directory with `.cargo/config.toml`)
+fn locate_workspace_root() -> Option<PathBuf> {
+    let mut dir = env::current_dir().ok()?;
+    while !dir.join(".cargo/config.toml").exists() {
+        if !dir.pop() {
+            return None;
+        }
+    }
+    Some(dir)
+}