Makefile: remove redundant mkdir from *config

The first dependency of these targets is
$(BUILD_DIR)/buildroot-config/*conf
so the $(BUILD_DIR)/buildroot-config directory certainly exists.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>