tools: Provide a defualt RUSTELFFLAGS if not defined

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>
diff --git a/Application.mk b/Application.mk
index da7dada..611355a 100644
--- a/Application.mk
+++ b/Application.mk
@@ -93,9 +93,10 @@
   OBJS += $(MAINCOBJ) $(MAINCXXOBJ) $(MAINRUSTOBJ) $(MAINZIGOBJ)
 endif
 
-# Compile flags
+# Compile flags, notice the default flags only suitable for flat build
 
 ZIGELFFLAGS ?= $(ZIGFLAGS)
+RUSTELFFLAGS ?= $(RUSTFLAGS)
 
 DEPPATH += --dep-path .
 DEPPATH += --obj-path .