Add multiboot + recovery/update partition

[jaxxzer]

As an alternative to the update process, we can have a small recover/update partition that we can boot into. This partition would remain completely static, and serve a webpage that allows uploading a new image file. The recovery partition will image a second (main) partition with the companion software image.

This method of updates will provide a lot of benefits:

• safer (can't brick companion)
• more consistent (check partition hash, no failed downloads)
• does not depend on wifi
• maintainable (does not require maintaining/bugfixing scripts that install dependencies)
• better application longevity (like a bootloader, the updating software/implementation is completely independent of the main/companion software)

Some things to think about:

• will this work on any companion computer? If so, are there any additional requirements on certain systems (ie two storage mediums, or differences in software approach)?

[patrickelectric]

• [x] Multiboot working
  1. Boot partition to manage boot order
  2. Recovery partition with clean debian operating system plus a simple webserver to manage the recovery and installation of the new companion image.
  3. Partition as destination for the companion image installation

Next steps with the multiboot is like this:

• Add an option in companion to reboot into update mode.
• Add an updater image that runs a "Recovery web server".
• The "Recovery web server" needs to have an interface for you to upload a compressed companion rootfs image to save to the raspberry pi disk.
• The "Recovery web server" needs to extracts the compressed linux image over top of the existing old image.
• Companion reboots into the updated partition.
• If there is any failure to boot into the main companion partition, always revert to boot into the updater partition running the "Recovery web server".
• Figure out handling of user data or configuration files so they are not overwritten.
• Maybe: Add a write protection on the updater partition

[jaxxzer]

For user data and configuration we can have yet another partition that is just for this, and remains untouched during the update process.