uefi: Deprecate helpers::system_table

This functionality is available with `table::system_table_boot` now.

Author: nicholasbishop

Diff for: uefi/CHANGELOG.md

@@ -19,6 +19,7 @@
   documentation for details of obligations for callers.
 - `BootServices::allocate_pool` now returns `NonZero<u8>` instead of
   `*mut u8`.
+- `helpers::system_table` is deprecated, use `table::system_table_boot` instead.
 
 ## Removed
 - Removed the `panic-on-logger-errors` feature of the `uefi` crate. Logger

Diff for: uefi/src/helpers/mod.rs

@@ -40,7 +40,7 @@ mod println;
 ///
 /// The returned pointer is only valid until boot services are exited.
 #[must_use]
-// TODO do we want to keep this public?
+#[deprecated(note = "use uefi::table::system_table_boot instead")]
 pub fn system_table() -> SystemTable<Boot> {
     table::system_table_boot().expect("boot services are not active")
 }