owut: allow to append apk repositories

[a-gave]

This allows to add apk repositories to the owut build requests.

[efahl]

Looks good! I'll give it a thorough review probably tomorrow.

Does this resolve #42? If so, could you add a "Fixes:" tag to the commit message.

Also, could you add a quick how-to in the commit message? Need not be long, just an example like below, with a little explanation of how the aliases tie into things?

First set up your custom feeds so that the work with apk:

echo "https://my.repo/packages.adb" >> /etc/apk/repositories.d/customfeeds.list
wget https://my.repo/my-keys.pem -O /etc/apk/keys/my-keys.pem

Then, to add them to owut upgrades, edit /etc/config/attendededsysupgrade and change it to

config owut 'owut'
  ...
  option repositories_mode  'append'
  list   repository_aliases '???'