Makefile: ensure $BINARIES_DIR exist before post-image scripts

When no filesystem is enabled, the $BINARIES_DIR is not created. Yet,
the post-image scripts are still run. When those want to generate an
image in there, they may fail as the dirctory does not exist (it did
exist before we started applying preparatory changes for top-level
parallel build, so scripts got to rely on that assumption).

Do in target-post-image as we do in the sdk rule: create the directory
before calling the scripts.

Signed-off-by: Brent Generous <bgenerous@impinj.com>
[yann.morin.1998@free.fr:
  - create the directory before calling the scripts
  - don't drop the creation in the sdk rule
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

Makefile

@@ -810,6 +810,7 @@ endif # merged /usr
 .PHONY: target-post-image
 target-post-image: $(TARGETS_ROOTFS) target-finalize staging-finalize
 	@rm -f $(ROOTFS_COMMON_TAR)
+	$(Q)mkdir -p $(BINARIES_DIR)
 	@$(foreach s, $(call qstrip,$(BR2_ROOTFS_POST_IMAGE_SCRIPT)), \
 		$(call MESSAGE,"Executing post-image script $(s)"); \
 		$(EXTRA_ENV) $(s) $(BINARIES_DIR) $(call qstrip,$(BR2_ROOTFS_POST_SCRIPT_ARGS))$(sep))