Add the rust std as a custom sysroot

crates/bffs/src/lib.rs

@@ -1,4 +1,5 @@
 #![feature(iterator_try_collect, iter_order_by)]
+#![allow(unused_features)]
 #![cfg_attr(any(not(feature = "std"), target_arch = "riscv64"), no_std)]
 
 use core::cell::RefCell;