fix(boot): tidy the devicetree, reboot and profile handling after review

Only warn about a devicetree under Secure Boot when the entry has one,
skip the fixup if its protocol can't be opened, and reject a devicetree
whose size is smaller than its header. Announce a reboot before the
delay, advertise the multi-profile UKI feature, and treat a Type 1 entry
with a profile after the first as an extra profile that is not picked as
the fallback default. A failure to read Sprout's own partition no longer
drops the entries of the XBOOTLDR partition, and the README lists every
initrd slot and describes the reboot behaviour more exactly.

crates/eficore/src/bootloader_interface.rs

@@ -68,6 +68,7 @@ impl BootloaderInterface {
             | LoaderFeatures::EntryPreferred
             | LoaderFeatures::SortKey
             | LoaderFeatures::DeviceTree
+            | LoaderFeatures::MultiProfileUki
     }
 
     /// Tell the system that Sprout was initialized at the current time.

crates/eficore/src/devicetree.rs

@@ -67,8 +67,8 @@ impl DeviceTree {
         if magic != FDT_MAGIC {
             bail!("the file is not a flattened devicetree");
         }
-        if total > dtb.len() {
-            bail!("the devicetree is truncated");
+        if total < FDT_HEADER_SIZE || total > dtb.len() {
+            bail!("the devicetree has an invalid size");
         }
         Ok(())
     }
@@ -127,8 +127,11 @@ impl DeviceTree {
         let Some(handle) = find_handle(&DT_FIXUP_GUID)? else {
             return Ok(());
         };
-        let mut protocol = uefi::boot::open_protocol_exclusive::<DtFixupProtocol>(handle)
-            .context("unable to open the devicetree fixup protocol")?;
+        // Without the protocol the devicetree is used as it is, as in systemd-boot.
+        let Ok(mut protocol) = crate::handle::open_shared::<DtFixupProtocol>(handle) else {
+            warn!("unable to open the devicetree fixup protocol, skipping the fixup");
+            return Ok(());
+        };
         let fixup = protocol.fixup;
         let this: *mut DtFixupProtocol = &mut *protocol;
         let flags = DT_APPLY_FIXUPS | DT_RESERVE_MEMORY;