Embedded Linux – mechanism for deploying firmware updates?

I am considering developing on the Yocto project for an embedded Linux
project (an industrial application) and I have a few questions for those
with experience with embedded Linux in general -- Yocto experience a
bonus.  Just need to get an idea of what is being commonly done in
firmware updates.

I have a few requirements, that being authentication, a secure
communications protocol, some type of rollback if the update failed. 
Also, if there is a way to gradually release the patch across the fleet of
devices then that would also be interesting as I want to avoid bricked
devices in the field.

How do you deploy updates/patches to field devices today – and how long
did it take to develop it?  Are there any other considerations I am
missing?  



---------------------------------------
Posted through http://www.EmbeddedRelated.com