Add the ability to flash all the slots (partitions) within one command using the same bundle

[Paiusco]

Problem: Currently, for bootstrapping from an image that already follows the same partitioning as the production image to both the slot groups, using RAUC could be more straight forward.

If we consider that one have an A+B system with a rootfs and a vfat for each. And all that one need is to flash the same bundle to both A and B, the process becomes a bit more cumbersome than what's needed if only the "other" needs to be updated. (rauc install http(s)://... suffices)

Alternative: After a very appreciated help from @ejoerns I got to the steps of:

• Mounting the bundle (rauc mount http(s):/...)
• Using write-slot for each one of the slots I have. Mentioning the correct name of each of the images inside the bundle (also error-prone)

Nice solution: IMO a nice solution would be to have an option even inside the install command where within a flag, I could force the same bundle to be installed in both the slot groups defined. (It might be cases where there are more slots that wouldn't accept this bundle tho, but maybe this is already considered within install) That would make all the previous mentioned steps become just one.

NOTE: It was mentioned that bootstrapping was previously argued to be out-of-scope, but maybe this one could still fit in the current concept.

NOTE2: If this is still considered to be out-of-scope and the alternative solution is more than acceptable, then I'd consider adding something to the documentation regarding this. I could do it, if we end up with this proposal.