uSDHC Hotplugging

[midnightveil]

• Virtualiser reading the MBR and assigning partitions with a static partition index <-> client index mapping is split out into a separate virtualiser policy interface, implemented by mbr.c (& virt.h)
• Added a hotplugging example that plays a "game" with a "safe" ejection state (see its README)
• Split out blk_storage_info_t to a storage_info.h header with some helper functions
• New interclient hotplug interface (sddf/hotplug/clients.h) and driver control interface (sddf/hotplug/control.h)
• uSDHC driver polls GPIO pins for card detection

To-Do:

• Design document (in progress) added to design doc
• use gpio driver when it exists
• integration with sdfgen tooling

[midnightveil]

Realised that I probably should port the other users of the block queues to send & receive the storage info ready channel?

At least, the drivers -- clients can busy poll if they don't care about the notification, but the virtualiser relies on the notifications now (and would also invoke null caps to notify the clients but that's a no-op)

[midnightveil]

Old code is https://github.com/au-ts/sddf/tree/sdhc-hotplugging-pre-sdfgen, in case the rebase broke anything. Should be deleted before merging.

I've rebased this on recent sddf changes, though sdfgen needs updates because of the extra channels.