Add `mkdir build` in makefile.
It this is missing, building with a clean repository will fail
because build/ does not exist.
@@ -143,6 +143,7 @@ $(kernel): kernel $(assembly_object_files) $(linker_script)
$(assembly_object_files) target/x86_64-blog_os/$(mode)/libucore.a
kernel:
+ @mkdir build/$(arch) -p
@CC=$(cc) cargo xbuild $(build_args)
# compile assembly files
