Rename UPDATE_AB_PARTITIONS to AB_OTA_PARTITIONS.

In order to have all the A/B updater variables in the same AB_OTA_
namespace we rename UPDATE_AB_PARTITIONS to AB_OTA_PARTITIONS.

Bug: None
Test: `make dist` includes the partitions.txt file if properly configured.

Change-Id: Ied4faabd7e2f0ab8a5007afb44ad2f38df8d630d
(cherry picked from commit 49be2b11)