build: consolidate directory creation rules

This commit streamlines directory creation by introducing a single
pattern rule to automatically make directories for which there is a
dependency.

We currently use several macros to generate rules to create directories
upon dependence, which is a significant amount of code and a lot of
redundancy. The rule introduced by this change represents a catch-all:
any rule dependency on a path ending in a forward slash is automatically
created.

Now, rules can rely on an unordered dependency (`|`) on `$$(@D)/` which,
when secondary expansion is enabled, expands to the directory of the
target being built, e.g.:

    build/main.o: main.c | $$(@D)/ # automatically creates `build/`

Change-Id: I7e554efa2ac850e779bb302fd9c7fbb239886c9f
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/plat/st/stm32mp1/platform.mk b/plat/st/stm32mp1/platform.mk
index a1f44e8..138f16c 100644
--- a/plat/st/stm32mp1/platform.mk
+++ b/plat/st/stm32mp1/platform.mk
@@ -255,11 +255,11 @@
 
 ifeq ($(AARCH32_SP),sp_min)
 # Create DTB file for BL32
-${BUILD_PLAT}/fdts/%-bl32.dts: fdts/%.dts fdts/${BL32_DTSI} | ${BUILD_PLAT} fdt_dirs
+${BUILD_PLAT}/fdts/%-bl32.dts: fdts/%.dts fdts/${BL32_DTSI} | $$(@D)/
 	$(q)echo '#include "$(patsubst fdts/%,%,$<)"' > $@
 	$(q)echo '#include "${BL32_DTSI}"' >> $@
 
-${BUILD_PLAT}/fdts/%-bl32.dtb: ${BUILD_PLAT}/fdts/%-bl32.dts
+${BUILD_PLAT}/fdts/%-bl32.dtb: ${BUILD_PLAT}/fdts/%-bl32.dts | $$(@D)/
 endif
 
 include plat/st/common/common_rules.mk