spi-api: don't silently hide config errors

This build script was throwing away any failure to parse global config,
which means that e.g. misspelling "DIV256" would instead get you
unrelated compile errors in apparently unrelated crates, since code
generation would just switch off.

This appears to have been done because the lpc55 code was depending on
this crate, and it of course doesn't _have_ an stm32h7 global SPI config
stanza in its config TOML.

I broke that dependency in an earlier commit, so now I'm making the code
generation mandatory with error reporting.

Author: cbiffle

drv/spi-api/build.rs

@@ -17,21 +17,17 @@ fn main() -> Result<()> {
     let dest_path = out_dir.join("spi_devices.rs");
     let mut file = File::create(dest_path)?;
 
-    if let Ok(global_config) = build_util::config::<SpiGlobalConfig>() {
-        writeln!(&mut file, "pub mod devices {{")?;
-        for (periph, p) in global_config.spi {
-            writeln!(
-                &mut file,
-                "    // {periph} ({} devices)",
-                p.devices.len()
-            )?;
-            for (i, name) in p.devices.keys().enumerate() {
-                let name = name.to_uppercase();
-                writeln!(&mut file, "    pub const {name}: u8 = {i};")?;
-            }
+    let global_config = build_util::config::<SpiGlobalConfig>()?;
+
+    writeln!(&mut file, "pub mod devices {{")?;
+    for (periph, p) in global_config.spi {
+        writeln!(&mut file, "    // {periph} ({} devices)", p.devices.len())?;
+        for (i, name) in p.devices.keys().enumerate() {
+            let name = name.to_uppercase();
+            writeln!(&mut file, "    pub const {name}: u8 = {i};")?;
         }
-        writeln!(&mut file, "}}")?;
     }
+    writeln!(&mut file, "}}")?;
 
     Ok(())
 }