[FS#973] ar71xx: sysupgrade broken on r4354 55623a9c83

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Ryan Mounce (rmounce) 

Attached to Project - LEDE Project
Summary - ar71xx: sysupgrade broken on r4354 55623a9c83
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Critical
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - TP-Link Archer C7 v2 running LEDE trunk r4354 55623a9c83

Sysupgrade fails with the message "Command failed: Method not found" on the following ubus call at the end of /sbin/sysupgrade

ubus call system sysupgrade "{
		\"prefix\": $(json_string "$RAM_ROOT"),
		\"path\": $(json_string "$IMAGE"),
		\"command\": $(json_string "$COMMAND")
	}"

Steps to reproduce:

-Checkout 55623a9c83, build a generic ar71xx NOR device and flash without saving config.
-SSH in and wget a newer sysupgrade image to /tmp/
-Run sysupgrade e.g. for debugging
# sh -x /sbin/sysupgrade -v /tmp/lede-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin
-Observe failure

In this case, passed arguments are
ubus call system sysupgrade {
		"prefix": "/tmp/root",
		"path": "/tmp/lede-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin",
		"command": ". /lib/functions.sh; include /lib/upgrade; do_upgrade_stage2"
	}


More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=973