Makefile: Ensure tools directory exists in output prior to use

With the change to make tools/version.h a file we need to make sure that
the output directory exists first otherwise we will get a build failure.

Reported-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Fixes: 4d90f6cd ("tools: Avoid creating symbolic links for tools/version.h")
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>