Build system: implement REUSE with SPDX identifiers

The SPDX identifiers [1] are a legally binding shorthand, which can be
used instead of the full boiler plate text. This is the first step
towards implementing the full REUSE spec [2] to help with copyright and
licensing audits and compliance.

This will reduce a lot a manual work required for the licensing audit
required in Debian on each update.

For files that lacked copyright and licensing information, I used the
following guidelines. If a clear author could be determined from the git
history use it, otherwise use 'EfficiOS Inc.'. For build system files,
use 'MIT', for documentation 'CC-BY-4.0' and for data files 'CC0-1.0'.

Freeform text files were converted to Markdown to allow licensing
comments.

[1] https://spdx.org/ids-how
[2] https://reuse.software/tutorial/

Change-Id: I75f6120ff1241e1f6ea5aac44cd87c89d7fd21e3
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>