feat(initrd): support chaining multiple initrds (#101)

Add linux-initrd-chain to the chainload action and initrd-chain to the
edera action. Each file in the list is read in order and concatenated
into a single initrd. The chain can't be combined with the single
initrd option on the same action.

BLS entries with multiple initrds, either as repeated initrd lines or
as multiple paths on one line, previously only loaded the last one.
Parse all of them and set them in the initrd-0 through initrd-7 slots,
which the autoconfigured BLS action now chains. Unused slots stamp to
the filesystem root and are skipped. Entries with more than 8 initrds
are skipped with a warning.

crates/boot/src/autoconfigure/bls.rs

@@ -1,3 +1,4 @@
+use crate::generators::bls::BLS_INITRD_SLOTS;
 use alloc::string::ToString;
 use alloc::{format, vec};
 use anyhow::{Context, Result};
@@ -83,10 +84,15 @@ pub fn scan(
 
     // Generate a chainload configuration for BLS.
     // BLS will provide these values to us.
+    // Every initrd slot is chained in order. Unused slots stamp to the root of the
+    // filesystem, which the chainload action skips.
     let chainload = ChainloadConfiguration {
         path: format!("{}\\$chainload", root),
         options: vec!["$options".to_string()],
-        linux_initrd: Some(format!("{}\\$initrd", root)),
+        linux_initrd: None,
+        linux_initrd_chain: (0..BLS_INITRD_SLOTS)
+            .map(|slot| format!("{}\\$initrd-{}", root, slot))
+            .collect(),
     };
 
     // Insert the chainload action into the configuration.