Document procedure to generate ELF binaries used in unit tests

[aljimenezb]

As discussed in #29, it is useful to have the instructions for generating the ELF binaries used for unit tests. Currently those instructions appear in the commits that introduce the binaries e.g. 0ce5bfa

Possible approaches are:

• add a section on the main README
• create a separate document
• generate the binaries at build time using Rust inline assembly

[alxiord]

How about a build hook that generates them, similar to how the bzimage test payload is obtained? For that, this script is run before the unit tests for bzimage. Can the build steps be packaged in a shell script and similarly included in the build?

LE: there's also a section in the readme on how to build the test binaries.

[andreeaflorescu]

We could also have this in two steps:

1. Add a section in a separate doc
2. Actually generate those binaries at build time

WDYT?

[alxiord]

Item 1 is a natural predecessor to 2, 2 could be picked up by any contributor who has the instructions, I like it.