Makefile: Create the build directory before bootblock.bin

This was causing a failure when building platforms with no bootblock
when building with make -jXX

Change-Id: Ic4cd4fe8ac82bd1e9ce114dbd53763538d125af3
Signed-off-by: Martin Roth <martinroth@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35531
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
diff --git a/Makefile.inc b/Makefile.inc
index 3c3088d..8676404 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -504,7 +504,7 @@
 	@printf "    GEN        build.h\n"
 	mv $< $@
 
-build-dirs:
+build-dirs $(objcbfs) $(objgenerated):
 	mkdir -p $(objcbfs) $(objgenerated)
 
 #######################################################################
@@ -706,7 +706,7 @@
 	$(OBJCOPY_bootblock) -O binary $< $@
 
 ifneq ($(CONFIG_HAVE_BOOTBLOCK),y)
-$(objcbfs)/bootblock.bin:
+$(objcbfs)/bootblock.bin: $(objcbfs)
 	dd if=/dev/zero of=$@ bs=64 count=1
 endif