firmware/shim: avoid mkdir error during Xen tree setup

firmware/shim: avoid mkdir error during Xen tree setup

"mkdir -p" reports a missing operand, as config/ has no subdirs. Oddly
enough this doesn't cause the whole command (and hence the build to
fail), despite the "set -e" now covering the entire set of commands -
perhaps a quirk of the relatively old bash I've seen this with (a few
simple experiments suggest that commands inside () producing a non-
success status would exit the inner shell, but not the outer one).

Add a dummy . argument to the invocation.

Suggested-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>