More Config refactoring

This is extracted from aosp/2874014 - these changes seem useful in
their own right.

The main aim is to make adding new entries easier.

Changes:
- Remove mut from entries we never need to change.
- Stop using deprecated zerocopy LayoutVerified.
- Extract ALL_ENTRIES constant so it's easy to spot.
- Simplify get_entries.
- Remove an unsafe block and use safe code instead.
- get_entries now consumes self (it can't now be called twice,
  but we don't need to).

Bug: 291232226
Test: TH
Change-Id: Ia2997cef94d3c506885ec7b35b9a4270a3e8542a
diff --git a/pvmfw/src/fdt.rs b/pvmfw/src/fdt.rs
index 2cd1061..0ead435 100644
--- a/pvmfw/src/fdt.rs
+++ b/pvmfw/src/fdt.rs
@@ -827,7 +827,7 @@
     bcc: &[u8],
     new_instance: bool,
     strict_boot: bool,
-    debug_policy: Option<&mut [u8]>,
+    debug_policy: Option<&[u8]>,
     debuggable: bool,
     kaslr_seed: u64,
 ) -> libfdt::Result<()> {