Add ro.build.system_root_image

ro.build.system_root_image is true if the root directory is built
into the system image.

Change-Id: I17208ebd004c63aebb09db6c3e5f795619e73bd2
diff --git a/core/Makefile b/core/Makefile
index 31cef3f..8ef0e39 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -203,6 +203,7 @@
 			BUILD_ID="$(BUILD_ID)" \
 			BUILD_DISPLAY_ID="$(BUILD_DISPLAY_ID)" \
 			BUILD_NUMBER="$(BUILD_NUMBER)" \
+			BOARD_BUILD_SYSTEM_ROOT_IMAGE="$(BOARD_BUILD_SYSTEM_ROOT_IMAGE)" \
 			PLATFORM_VERSION="$(PLATFORM_VERSION)" \
 			PLATFORM_SECURITY_PATCH="$(PLATFORM_SECURITY_PATCH)" \
 			PLATFORM_BASE_OS="$(PLATFORM_BASE_OS)" \