Use C string literals not cstr!

Since Rust 1.77.0 the language has supported C string literals, so the
`cstr!` macro is no longer needed. Replace existing usages with the
equivalent literal.

See https://doc.rust-lang.org/reference/tokens.html#c-string-literals.

I believe that the two are equivalent:
- Escapes are handled the same way;
- Both allow arbitrary Unicode, which is mapped to UTF-8 (I don't
  think we made any use of this);
- Both treat any embedded NUL character as a compile time error.

This is of no significance whatsoever, but it does make the code a
tiny bit simpler. It should not change the compiled code at all, so
no flagging should be needed.

I'm not deleting the macro in this CL; I'll do a follow-up for that,
since there may be usages I can't see, and it has greater chance of
accidental conflict.

Test: TH
Change-Id: I4354b3b0a0c53fbec0c2d78b4182786e4e2d0ce8
diff --git a/guest/rialto/src/fdt.rs b/guest/rialto/src/fdt.rs
index e97a262..06879d0 100644
--- a/guest/rialto/src/fdt.rs
+++ b/guest/rialto/src/fdt.rs
@@ -15,24 +15,23 @@
 //! High-level FDT functions.
 
 use core::ops::Range;
-use cstr::cstr;
 use libfdt::{Fdt, FdtError};
 
 /// Reads the DICE data range from the given `fdt`.
 pub fn read_dice_range_from(fdt: &Fdt) -> libfdt::Result<Range<usize>> {
-    let node = fdt.node(cstr!("/reserved-memory"))?.ok_or(FdtError::NotFound)?;
-    let node = node.next_compatible(cstr!("google,open-dice"))?.ok_or(FdtError::NotFound)?;
+    let node = fdt.node(c"/reserved-memory")?.ok_or(FdtError::NotFound)?;
+    let node = node.next_compatible(c"google,open-dice")?.ok_or(FdtError::NotFound)?;
     node.first_reg()?.try_into()
 }
 
 pub(crate) fn read_vendor_hashtree_root_digest(fdt: &Fdt) -> libfdt::Result<Option<&[u8]>> {
-    let node = fdt.node(cstr!("/avf"))?.ok_or(FdtError::NotFound)?;
-    node.getprop(cstr!("vendor_hashtree_descriptor_root_digest"))
+    let node = fdt.node(c"/avf")?.ok_or(FdtError::NotFound)?;
+    node.getprop(c"vendor_hashtree_descriptor_root_digest")
 }
 
 pub(crate) fn read_is_strict_boot(fdt: &Fdt) -> libfdt::Result<bool> {
     match fdt.chosen()? {
-        Some(node) => Ok(node.getprop(cstr!("avf,strict-boot"))?.is_some()),
+        Some(node) => Ok(node.getprop(c"avf,strict-boot")?.is_some()),
         None => Ok(false),
     }
 }