Add ota script support to generate partial updates

Allow the ota generation script to take a list of partitions;
and thus generating a partial ota update package. This allow the
testing of partial update functionality, e.g. system-only updates.

Bug: 170921953
Test: unit tests, generate and apply a partial update on coral with system, system_ext, vbmeta_system
Change-Id: I0d8f93806dfbf7b781ea911117b02fc5c0971434