Update the dtc submodule in configure already and symlink dtc after
git submodule update, because on win32 symlinks to non-existing folders
are forbidden.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20200902170054.810-2-luoyonggang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>