kirkwood: Fix FS#505 - Can't install LEDE on Linksys EA3500

The default sizes render Device/linksys_audi mage un-flashable.
Restore the pagesize, subpagesize, and blocksize for linksys_audi
from https://github.com/openwrt/archive.

Signed-off-by: Eubert Bao <bunnier@gmail.com>
(cherry picked from commit e11fc8439c)

target/linux/kirkwood/image/Makefile

@@ -82,6 +82,9 @@ define Device/linksys_audi
   DEVICE_TITLE := Linksys EA3500 (Audi)
   DEVICE_PACKAGES := kmod-mwl8k swconfig wpad-basic kmod-gpio-button-hotplug
   DEVICE_DTS := kirkwood-linksys-audi
+  PAGESIZE := 512
+  SUBPAGESIZE := 256
+  BLOCKSIZE := 16KiB
   KERNEL_SIZE := 2624k
   KERNEL_IN_UBI :=
   UBINIZE_OPTS := -E 5