Export ROM_BUILDTYPE to soong

Change-Id: Ie66cee2ea5caf0fdb567791d9fd439b854e480cb
diff --git a/core/soong_extra_config.mk b/core/soong_extra_config.mk
index 6f370c5..6765813 100644
--- a/core/soong_extra_config.mk
+++ b/core/soong_extra_config.mk
@@ -46,6 +46,7 @@
 
 $(call add_json_list, OMNI_PRODUCT_PROPERTIES,      $(call collapse-prop-pairs,OMNI_PRODUCT_PROPERTIES))
 $(call add_json_list, PRODUCT_BUILD_PROP_OVERRIDES,      $(call collapse-prop-pairs,PRODUCT_BUILD_PROP_OVERRIDES))
+$(call add_json_list, ROM_BUILDTYPE,      $(call collapse-prop-pairs,ROM_BUILDTYPE))
 
 $(call add_json_str, BootloaderBoardName, $(TARGET_BOOTLOADER_BOARD_NAME))