build: Make sure we get the correct previous tag for the changelog

When going from an -rc release to a stable release (e.g. v0.5.8 ;-) the
generated shortlog will not be generated from the latest -rc tag but
from the second to last due to the sort order of the tag list:

0.5.5
0.5.6
0.5.7
v0.5.8      <-- newest tag
v0.5.8-rc1
v0.5.8-rc2
v0.5.8-rc3
v0.5.8-rc4
v0.5.8-rc5  <-- second newest tag

Thus use 'git describe' to get the proper previous tag.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
conf: oui: update oui.conf

Update oui.conf using oui-update.py

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
build: Also use proper previous tag for people list

Follow up for commit 311d4376 ("build: Make sure we get the correct
previous tag for the changelog").

Move getting the previous tag into an own command and use it where
appropriate.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>