buildroot

docs/manual: refer to SPDX License List

We want to use SPDX short identifier for license string as much as
possible. So refer to SPDX license list and use SPDX short identifier
for license strings in documentation.

Also remove License abbreviations that we have.

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
[Thomas: take into account suggestions from Arnout.]
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>