Do not overwrite user configuration

package/Makefile.in

@@ -10,19 +10,19 @@ MAKE1:=$(HOSTMAKE) -j1
 MAKE:=$(HOSTMAKE) -j$(BR2_JLEVEL)
 
 ifeq ($(BR2_OPTIMIZE_0),y)
-TARGET_OPTIMIZATION=-O0
+TARGET_OPTIMIZATION+=-O0
 endif
 ifeq ($(BR2_OPTIMIZE_1),y)
-TARGET_OPTIMIZATION=-O1
+TARGET_OPTIMIZATION+=-O1
 endif
 ifeq ($(BR2_OPTIMIZE_2),y)
-TARGET_OPTIMIZATION=-O2
+TARGET_OPTIMIZATION+=-O2
 endif
 ifeq ($(BR2_OPTIMIZE_3),y)
-TARGET_OPTIMIZATION=-O3
+TARGET_OPTIMIZATION+=-O3
 endif
 ifeq ($(BR2_OPTIMIZE_S),y)
-TARGET_OPTIMIZATION=-Os
+TARGET_OPTIMIZATION+=-Os
 endif
 ifeq ($(BR2_DEBUG_1),y)
 TARGET_DEBUGGING=-g1