plat: marvell: armada: Add missing FORCE, .PHONY and clean targets

FORCE target is used as a dependency for other file targets which needs to
be always rebuilt. .PHONY target is standard Makefile target which specify
non-file targets and therefore needs to be always rebuilt.

Targets clean, realclean and distclean are .PHONY targets used to remove
built files. Correctly set that mrvl_clean target is prerequisite for these
clean targets to ensure that built files are removed.

Finally this change with usage of FORCE target allows to remove mrvl_clean
hack from the prerequisites of a8k ${DOIMAGETOOL} target which was used
just to ensure that ${DOIMAGETOOL} is always rebuilt via make subprocess.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I2fa8971244b43f101d846fc433ef7b0b6f139c92
diff --git a/plat/marvell/armada/common/marvell_common.mk b/plat/marvell/armada/common/marvell_common.mk
index 7f8dffa..9d4cca1 100644
--- a/plat/marvell/armada/common/marvell_common.mk
+++ b/plat/marvell/armada/common/marvell_common.mk
@@ -93,4 +93,5 @@
 	@truncate -s %4 $(BUILD_PLAT)/$(BOOT_IMAGE) || { rm -f $(BUILD_PLAT)/$(BOOT_IMAGE); false; }
 	@echo "Built $@ successfully"
 
+.PHONY: mrvl_bootimage
 mrvl_bootimage: $(BUILD_PLAT)/$(BOOT_IMAGE)