fix(bls): allow BLS directories without an entries directory

Autoconfigure detects BLS when either \loader\loader.conf or
\loader\entries exists, but the BLS generator required the entries
directory and failed the boot without it. This broke ESPs that only
have a loader.conf, such as UKI-only systemd-boot setups. Generate no
entries when the directory is missing.

crates/boot/src/generators/bls.rs

@@ -60,6 +60,15 @@ pub fn generate(context: Rc<SproutContext>, bls: &BlsConfiguration) -> Result<Ve
             .context("unable to open bls filesystem")?;
     let mut fs = FileSystem::new(fs);
 
+    // The entries directory is optional, as a BLS directory may only contain a loader.conf file.
+    // If it does not exist, there are no entries to generate.
+    if !fs
+        .try_exists(&entries_path)
+        .context("unable to check for bls entries directory")?
+    {
+        return Ok(Vec::new());
+    }
+
     // Read the BLS entries directory.
     let entries_iter = fs
         .read_dir(&entries_path)