configure: Check mkdir result directly, not via $?

Shellcheck warns that we have one place where we run a command and
then check if it failed using $?; this is better written to simply
check the command in the 'if' statement directly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220825150703.4074125-7-peter.maydell@linaro.org