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

[LEDE Commits]

neoraider pushed a commit to source.git, branch master:
https://git.lede-project.org/e407f1a4c849dd5b4aac9ddd31d2542e45077847

commit e407f1a4c849dd5b4aac9ddd31d2542e45077847
Author: Matthias Schiffer <mschiffer at universe-factory.net>
AuthorDate: Tue Oct 11 20:32:25 2016 +0200

    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 at universe-factory.net>
---
 .../base-files/etc/uci-defaults/09_fix-seama-header     | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/target/linux/ar71xx/base-files/etc/uci-defaults/09_fix-seama-header b/target/linux/ar71xx/base-files/etc/uci-defaults/09_fix-seama-header
new file mode 100644
index 0000000..ebe9c1c
--- /dev/null
+++ b/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