Add boot crypto framework

[eric_metricspace.net]

Summary

This adds a crypto framework for boot-time code without the size restrictions to which BIOS code is subject. The framework is designed to allow new ciphers to be added relatively easily.

Note: it is probable that this code will be replaced in the mid-term future. There is talk of a crypto overhaul, but this will not take place in any timeline suitable for GELI integration.

Test Plan

This has already been tested for both AES-CBC and AES-XTS in testing of the GELI boot code. Its present form is unmodified from the last successful test.

[eric_metricspace.net]

Rebased to HEAD.

This one is independent of the other GELI precursors, and can go in as-is.

[eric_metricspace.net]

Update to reflect move to /stand

[eric_metricspace.net]

Rebase from HEAD

[eric_metricspace.net]

Rebase to HEAD

[eric_metricspace.net]

Alternate approach to GELI was merged. This is no longer needed.