Read-only root with data partition

[zehnm]

This pull request changes the partition setup and introduces a compressed read-only root file system. A writable data partition holds overlay data to persist configuration settings.

Partition	Size	Format	PARTUUID
Boot	32M	vfat	53a3720d-07aa-4680-ab6d-f2ed0979c9ea
Recovery	384M	ext4 (TBD)	b9ea076c-315f-422b-85d7-887854415b1e
System A	512M	SquashFS	b831b597-efc4-4132-b88c-c50a2d4589cf
System B	512M	SquashFS	f2f82015-3087-485a-9241-914026bca453
Data	128M	ext4	79055324-d7a8-4768-afc6-c7dbfc9a4612

• Use SquashFS as root file system.
• ext4 data partition to store individual bind-mounted configuration files in /etc
• OverlayFS for /var and /opt/yio which are persisted on the data partition.
• Temporary in-memory overlay for /etc to find all configuration settings which need to be bind-mounted to the data partition.
• Overlay file systems are setup in a custom init script.
  This is mainly a quick fix to get everything working with systemd. Setting up all overlays and bind-mounts with systemd might be introduced later, to allow proper file system checks and auto-expansion of the data partition.

Further changes:

• Hybrid GPT MBR partition setup to avoid dealing with extended partitions.
  • Partition UUIDs are used wherever possible to uniquely reference partitions, otherwise file system UUID.
  • Partition numbers are now only used in the U-Boot boot script to load the correct kernel.
    This changes the SWUpdate target handling: instead of using the hard coded partition number, the systems are referenced by A / B slots and their PARTUUID. If the partition layout changes, then only the U-Boot boot script needs to be changed.

Open issues:

• Mongoose web server is running openly by default.
  Available options:
  • Configure authentication
  • Manually start / stop daemon or integrate in remote-software
  • Fully integrate into web-configurator
• Partition layout and sizes:
  • both 512MB rootfs partitions are rather generous for SquashFS
  • data partition at the end is kept small to make it auto expandable, which is not yet implemented
  • decide about data partition: keep as is / split into an overlay partition for configuration data and an app partition for all YIO components.
• systemd machine_id handling still throws an error during boot (but seems to work otherwise)
• Find out which configuration settings in /etc need to be persisted.
  Known settings not persisted at the moment (in-memory overlay):
  • timezone: requires /etc/localtime symlink to /usr/share/zoneinfo/{region}/{town}
  • public ssh keys cannot be persisted due to read-only /root/.ssh
• YIO app and plugin handling during system update
  • /opt/yio is an overlay and can still be updated with the old update scripts
  • A manually updated app or plugin is persisted during a system update and might become incompatible with a newly applied system update.
    There should be an option to wipe the overlay to have a clean update.
• A recovery system partition is prepared, but not yet functional.
  This will be handled in another task. Also the file system might change to SquashFS.

This closes #16
This closes #29
Part of #49 for a read-only SD card setup.