kernel: set lz4 compression level to 9

Change-Id: I34bfcb10c25bdca2a66ef9f8daaf3d64475d050b
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index 0e17fd8..c339000 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -373,7 +373,7 @@
 
 ifeq ($(BOARD_KERNEL_IMAGE_LZ4),true)
 $(file) : $(KERNEL_BIN)
-	$(LZ4) -f $(KERNEL_BIN) $@
+	$(LZ4) -9 -f $(KERNEL_BIN) $@
 else
 $(file) : $(KERNEL_BIN) | $(ACP)
 	$(transform-prebuilt-to-target)