Switch kernel builds away from using xargo

Use cargo's 'build-std=core,alloc' instead.

Also: Cargo is now given the complete path to the target specific JSON file.
This forces the use of the JSON file (the file is silently ignored if
the target in question was built into the rust compiler).

mk/prefix.mk

@@ -28,6 +28,8 @@ $(PREFIX)/relibc-install: $(ROOT)/relibc | $(PREFIX)/rust-install
 	cd "$@.partial" && $(PREFIX_STRIP)
 	touch "$@.partial"
 	mv "$@.partial" "$@"
+	mkdir $@/lib/rustlib/src
+	ln -s $(ROOT)/rust $@/lib/rustlib/src
 
 $(PREFIX)/relibc-install.tar.gz: $(PREFIX)/relibc-install
 	tar \