`vault` app release

This release of Xous includes the vault app, as well as major ergonomic changes to the device update flow. It also introduces the ability to make backups of the device.

v0.9.6 - backer shipment MVP

This version of Xous is the MVP milestone for backer shipment. It is strongly recommended that all backers who receive their devices update to this version before personalizing their units, as there are several API-breaking fixes in this release compared to what was flashed in the factory several months ago.

While we are striving to stabilize all the APIs, we're still not at 1.0.0, which means we expect that we'll find bugs that could require breaking backward compatibility with provisioned/root-keyed devices. Breaking backward compatibility means requiring a JTAG-cable level "unbrick" process to bring things up to date, that would also wipe any data from the device. An example of an API-breaking change that is on the books is hardware support for Bcrypt. Right now we are limited to a work factor of 7, but OWASP recommends 10. 10 is suitable for GHz-class servers, but on Precursor it would take tens of seconds to compute a login password hash. Thus, there is an open item to investigate the possibility of hardware acceleration for the Bcrypt primitive to increase the work factor. If this work factor is increased, then it would change the password security scheme and invalidate existing passwords.

This version is tagged out to a "release" because it has also been pushed to the factory, so future units that roll off the production line will be shipped with software built using this particular code base. This corresponds to the bootstrap-mainboard repo version v0.7.4 state.

Simplest end-to-end functionality

Interactive shell + sensors and power on/off.

Might as well tag it.