Rewrite: Remove support for bootimage {run, test}

[phil-opp]

This pull request rewrites the bootimage crate almost completely. The most notable changes are:

• Breaking: Removes support for the old bootimage run and bootimage test commands because they were difficult to maintain.
• Breaking: Removes support for the default-target config key because it did not play nice with cargo workspaces.
• Breaking: The crate structure was changed completely, so users who use bootimage as a library will need to update their code when updating to the new (semver-incompatible) API.
• Rewrite the code for finding the kernel Cargo.toml file to make bootimage work with cargo workspaces.
• Switches error handling to the anyhow/thiserror crates.
• Updates some dependencies.

Migration

Users of bootimage run should switch to the runner-based API provided by the bootimage runner sucommand as described here. Instead of bootimage test, custom test frameworks should be used together with cargo xtest. Instead of using the default-target config key, setting a default target is possible through a build.target key in a .cargo/config file.

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Fixes #54

[phil-opp]

Published as version 0.8.0