airoha: do not trash feeds/airoha with built/installed files

airoha/dir.mak

@@ -66,7 +66,7 @@ APP_BSP_DIR=$(TRUNK_DIR)/app_bsp
 APP_BSP_PRIVATEDIR=$(APP_BSP_DIR)/private
 APP_BSP_PUBLICDIR=$(APP_BSP_DIR)/public
 
-EXPORT_BSP_DIR=$(TRUNK_DIR)/install_bsp
+EXPORT_BSP_DIR=$(KERNEL_BUILD_DIR)/install_bsp
 INSTALL_BSP_DIR=$(EXPORT_BSP_DIR)
 export LIB_INSTALL_DIR=$(EXPORT_BSP_DIR)/bsp_install
 APP_BSP_LIB_DIR=$(LIB_INSTALL_DIR)