ar71xx: add back SEAMA header checksum fix (as used on ramips)

The D-Link DIR-869 A1 doesn't accept images with the jffs2 marker added
after the checksummed range, so we need to include it in the checksum and
fix it on first boot.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

target/linux/ar71xx/base-files/etc/uci-defaults/09_fix-seama-header

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+. /lib/ar71xx.sh
+
+fix_seama_header() {
+	local kernel_size=$(sed -n 's/mtd[0-9]*: \([0-9a-f]*\).*"kernel".*/\1/p' /proc/mtd)
+
+	[ "$kernel_size" ] && mtd -c 0x$kernel_size fixseama firmware
+}
+
+board=$(ar71xx_board_name)
+
+case "$board" in
+dir-869-a1)
+	fix_seama_header
+	;;
+esac