Add a hook for the syslinux bootloader.

Change-Id: Ia8638e9dcb025743a66c52ad43b161ee70602c89
diff --git a/core/Makefile b/core/Makefile
index 6c49580..de4a268 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -491,6 +491,11 @@
 tmp_dir_for_image := $(call intermediates-dir-for,EXECUTABLES,boot_img)/bootimg
 INTERNAL_BOOTIMAGE_ARGS += --tmpdir $(tmp_dir_for_image)
 INTERNAL_BOOTIMAGE_ARGS += --genext2fs $(MKEXT2IMG)
+
+ifeq ($(TARGET_BOOTIMAGE_USE_EXTLINUX),true)
+INTERNAL_BOOTIMAGE_ARGS += --extlinuxconf $(TARGET_BOOTIMAGE_EXTLINUX_CONFIG)
+endif
+
 $(INSTALLED_BOOTIMAGE_TARGET): $(MKEXT2IMG) $(INTERNAL_BOOTIMAGE_FILES)
 	$(call pretty,"Target boot image: $@")
 	$(hide) $(MKEXT2BOOTIMG) $(INTERNAL_BOOTIMAGE_ARGS) --output $@