image.in, main.mk: align debug targets

Weird but the last round of image builds on e2k started complaining:

  Makefile:95: *** target file `debug' has both : and :: entries.  Stop.

Looks like these should have been fixed indeed.
But why didn't this surface before then?

image.in/Makefile

@@ -92,7 +92,7 @@ postprocess: | $(addprefix postprocess-,$(sort $(POSTPROCESS_TARGETS)))
 		cp -a distcfg.mk "$(LOGDIR)/$(IMAGE_OUTFILE).cfg"; \
 	fi
 
-debug:
+debug::
 	@echo "TOPDIR=$(TOPDIR)"
 	@echo "IMAGEDIR=$(IMAGEDIR)"
 	@echo "ARCH=$(ARCH)"

main.mk

@@ -91,7 +91,7 @@ $(IMAGES): debug \
 # convenience shortcut
 $(DISTROS:distro/%=%): %: distro/%; @:
 
-debug:
+debug::
 ifeq (2,$(DEBUG))
 	@$(foreach v,\
 		$(filter IMAGE_%,$(sort $(.VARIABLES))),\