Update makefile

Makefile

@@ -1,2 +1,9 @@
 ucore:
-	@RUST_TARGET_PATH=$(shell pwd) xargo build --target ucore --features ucore
+	rustup override set nightly-2018-04-01
+	RUST_TARGET_PATH=$(shell pwd) xargo build --target ucore --features ucore
+
+install-rust:
+	which cargo || ( curl https://sh.rustup.rs -sSf | sh )
+	which xargo || ( rustup component add rust-src && cargo install xargo )
+
+.PHONY: ucore install-rust