fix upgrading from 2.1 stable to edge
When doing upgrade to edge from 2.1, when running apk update I get a:
Invalid index entry ‘A’
Means its not possible to upgrade to edge (without wget/tar).
We need backport the reading of ‘A’ field in index in 2.1 stable - but not add the A field to the 2.1 index.
(from redmine: issue id 537, created on 2011-03-01, closed on 2011-03-08)