uboot-ar71xx: fix build error

The HOSTCFLAGS override only works on newer u-boot versions

Signed-off-by: Felix Fietkau <[email protected]>

package/boot/uboot-ar71xx/Makefile

@@ -26,6 +26,8 @@ define U-Boot/nbg460n_550n_550nh
   HIDDEN:=y
 endef
 
+UBOOT_MAKE_FLAGS :=
+
 UBOOT_TARGETS:=nbg460n_550n_550nh
 
 $(eval $(call BuildPackage/U-Boot))