rust: str: allow dereferencing BStr in a const ctx

impl Deref doesn't work in const context. Add a function
that is similar to implementing `deref` but that can
be used in `const` context.

Signed-off-by: Fabien Parent <fabien.parent@linaro.org>

Author: Fabo

rust/kernel/str.rs

@@ -32,6 +32,12 @@ impl BStr {
         unsafe { &*(bytes as *const [u8] as *const BStr) }
     }
 
+    /// Returns a reference to the inner [u8].
+    #[inline]
+    pub const fn deref_const(&self) -> &[u8] {
+        &self.0
+    }
+
     /// Strip a prefix from `self`. Delegates to [`slice::strip_prefix`].
     ///
     /// # Examples
@@ -121,7 +127,7 @@ impl Deref for BStr {
 
     #[inline]
     fn deref(&self) -> &Self::Target {
-        &self.0
+        self.deref_const()
     }
 }