Revert "kernel: Remove unneeded MKDTIMG dependency"

This reverts commit caa3bddf00129c06f35923e43d5e770645da6b99.

Reason for revert: Breaks pixel dtbo.img build, since kernel depends on
mkdtimg. Add it back so it gets compiled and kernel can pick it up.

Change-Id: I2b9fd0b2b35e0cf2f908516b10f65399780d5c30
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index 9f17c3d..9bcbce3 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -438,12 +438,13 @@
 ifneq ($(BOARD_CUSTOM_DTBOIMG_MK),)
 include $(BOARD_CUSTOM_DTBOIMG_MK)
 else
+MKDTIMG := $(HOST_OUT_EXECUTABLES)/mkdtimg$(HOST_EXECUTABLE_SUFFIX)
 MKDTBOIMG := $(HOST_OUT_EXECUTABLES)/mkdtboimg.py$(HOST_EXECUTABLE_SUFFIX)
 
 $(DTBO_OUT):
 	mkdir -p $(DTBO_OUT)
 
-$(BOARD_PREBUILT_DTBOIMAGE): $(DTC) $(MKDTBOIMG) $(DTBO_OUT)
+$(BOARD_PREBUILT_DTBOIMAGE): $(DTC) $(MKDTIMG) $(MKDTBOIMG) $(DTBO_OUT)
 $(BOARD_PREBUILT_DTBOIMAGE):
 	@echo "Building dtbo.img"
 	$(hide) find $(DTBO_OUT)/arch/$(KERNEL_ARCH)/boot/dts -type f -name "*.dtbo" | xargs rm -f