kernel: add BOARD_KERNEL_IMAGE_LZ4 to run lz4 on kernel

uboot devices can decompress such images

Change-Id: Ic717b25d2323bb55c9a5bb0f5ce078583a0ef5ae
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index eafaa9f..abc6d98 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -354,8 +354,14 @@
 ifeq ($(NEEDS_KERNEL_COPY),true)
 file := $(INSTALLED_KERNEL_TARGET)
 ALL_PREBUILT += $(file)
+
+ifeq ($(BOARD_KERNEL_IMAGE_LZ4),true)
+$(file) : $(KERNEL_BIN)
+	$(LZ4) -f $(KERNEL_BIN) $@
+else
 $(file) : $(KERNEL_BIN) | $(ACP)
 	$(transform-prebuilt-to-target)
+endif
 
 ALL_PREBUILT += $(INSTALLED_KERNEL_TARGET)
 endif