build: make mkfs-* depend on kernel installation to fix parallel build issues

Reported-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 47296

include/image.mk

@@ -282,7 +282,7 @@ endef
 define BuildImage/mkfs
   install: mkfs-$(1)
   .PHONY: mkfs-$(1)
-  mkfs-$(1): mkfs_prepare
+  mkfs-$(1): kernel_prepare
 	$(Image/mkfs/$(1))
 	$(call Build/mkfs/default,$(1))
 	$(call Build/mkfs/$(1),$(1))