ar71xx: fix splitting firmware partition for TL-WR902AC v1

The -O option for the tplink-v1-header was missing for the TP-Link
TL-WR902AC v1, while safeloader and MTDPARTS where set up with a
single firmware partition.

This led to bootloops after using sysupgrade.

Fixes: FS#3118

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit a7b07f8ba880895f0d235a63729dd189cb2410a7)

target/linux/ar71xx/image/generic-tp-link.mk

@@ -525,6 +525,8 @@ define Device/tl-wr902ac-v1
   SUPPORTED_DEVICES := tl-wr902ac-v1
   IMAGE_SIZE := 7360k
   MTDPARTS := spi0.0:128k(u-boot)ro,7360k(firmware),640k(tplink)ro,64k(art)ro
+  KERNEL := kernel-bin | patch-cmdline | lzma | tplink-v1-header -O
+  KERNEL_INITRAMFS := kernel-bin | patch-cmdline | lzma | tplink-v1-header
 endef
 TARGET_DEVICES += tl-wr902ac-v1