plat-stm: fix parallel build error

Generation of out/arm32-plat-stm/core/tz.lds may occur when the directory
out/arm32-plat-stm/core does not exist yet. The command should therefore
create it.
Note: plat-vexpress is correct.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>