buildroot

docs/website: update sponsors for FOSDEM

Simply a matter of updating the year, since it's again Google and Mind
for the FOSDEM2018 developer meeting.

Also add the 2017 sponsorship to the "Past sponsors" section.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>