Fix override of CARGOFLAGS when using target

Makefile

@@ -1,12 +1,11 @@
 TARGET?=
 
 CARGO?=cargo
-CARGOFLAGS?=
 
 BUILD=target
 ifneq ($(TARGET),)
 	BUILD="target/$(TARGET)"
-	CARGOFLAGS+="--target=$(TARGET)"
+	CARGOFLAGS?="--target=$(TARGET)"
 endif
 
 ifeq ($(TARGET),aarch64-unknown-linux-gnu)