only build flash_image for eng

With the recovery image being installed by applypatch, the flash_image
tool isn't needed any more.  Continue to build it for eng just in case
it's handy for debugging.
diff --git a/mtdutils/Android.mk b/mtdutils/Android.mk
index 1a75423..57ab579 100644
--- a/mtdutils/Android.mk
+++ b/mtdutils/Android.mk
@@ -15,6 +15,7 @@
 include $(CLEAR_VARS)
 LOCAL_SRC_FILES := flash_image.c
 LOCAL_MODULE := flash_image
+LOCAL_MODULE_TAGS := eng
 LOCAL_STATIC_LIBRARIES := libmtdutils
 LOCAL_SHARED_LIBRARIES := libcutils libc
 include $(BUILD_EXECUTABLE)