uboot-fritz4040: build with ipq40xx "generic" subtarget

Fixes: 853e4dd3 ("ipqx0xx: add Generic subtarget")
Ref: https://forum.openwrt.org/t/ipq40xx-snapshot-not-updated-since-22nd-august/44126
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
(cherry picked from commit 40e3f660c1c0f400092cce09feb8c13bec97caeb)

package/boot/uboot-fritz4040/Makefile

@@ -21,6 +21,7 @@ include $(INCLUDE_DIR)/package.mk
 
 define U-Boot/Default
   BUILD_TARGET:=ipq40xx
+  BUILD_SUBTARGET:=generic
   UBOOT_BOARD:=$(1)
   UBOOT_IMAGE:=uboot-$(1).bin
 endef