[FS#315] Improve sysupgrade (CLI) on dual-partition routers

[LEDE Bugs]

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

User who did this - Stan (stangri) 

Attached to Project - LEDE Project
Summary - Improve sysupgrade (CLI) on dual-partition routers
Task Type - Feature Request
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Some of the recent Linksys (maybe other) routers have two partitions: WRT1900, EA8500, etc.

Would be great if when running sysupgrade on those routers it did NOT kill all active processes and kept the router working while writing an image to the inactive partition.

Would also be great if there was an option to prevent automatic reboot after writing an image.

Ultimate goal is to have an upgrade process where current working state is uninterrupted and only when you reboot -- you boot into new image.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=315