feat(bls): read the extended boot loader partition in the BLS generator

Add an xbootldr option to the BLS generator. When it is enabled, the
partition with the XBOOTLDR type on the same disk as Sprout's partition
is read as well, and its entries and unified kernel images are sorted
with the others. As their files are on that partition, every entry has
an entry-root value, which is the device of the partition or nothing for
Sprout's own, to use in a path like $entry-root\$chainload. It is off
by default, as existing actions would look for the files on the wrong
partition.

crates/boot/src/autoconfigure/bls.rs

@@ -102,6 +102,9 @@ pub fn scan(
         },
         path: format!("{}\\loader", root),
         uki_path: None,
+        // Every filesystem is scanned, including the Extended Boot Loader Partition, which
+        // gets a generator of its own.
+        xbootldr: false,
         pin_names: true,
     };