lib/rsa: don't use NULL as key_id

If keydir is not provided but name is we want to use name as key_id.

But with the current coding name is only used on its own if it is NULL
and keydir is provided which never occurs.

Fixes: 824ee745 ("lib/rsa: Use the 'keyfile' argument from mkimage")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>