Use cargo build instead of cargo xbuild

Reference:
  rust-lang/cargo#8393

Makefile

@@ -33,7 +33,7 @@ clean:
 #
 $(src)/rust/target/x86_64-linux-kernel/release/libflashbench.a: $(src)/rust/Cargo.toml $(wildcard $(src)/rust/src/*.rs)
 	cd $(src)/rust &&\
-	  env -u MAKE -u MAKEFLAGS cargo +nightly xbuild --release
+	  env -u MAKE -u MAKEFLAGS cargo +nightly build --release
 
 %.rust.o: rust/target/x86_64-linux-kernel/release/lib%.a
 	$(LD) -r -o $@ --whole-archive $<

rust/.cargo/config

@@ -1,5 +1,5 @@
-[alias]
-xbuild = "build -Z build-std=core"
-
 [build]
 target = "x86_64-linux-kernel"
+
+[unstable]
+build-std = ["core"]

rust/README.md

@@ -2,11 +2,11 @@ Rust part of flashbench
 ========
 
 ```bash
-# Use nightly as default toolchain in this directory
-rustup override set nightly
+cargo +nightly build
 
-# xbuild is alias of `build -Z build-std=core`
-cargo xbuild
+# You can set nightly as default rust toolchain in this project using rustup
+rustup override set nightly
+cargo build
 ```
 
 ### TODOs