Allow otatools to deduce partial partition list from input

Different devices/builds might have different constraints on what
partitions should be updated together in a partial OTA. Add a convenient
feature to auto detect.

Test: generate ota with --partial , make sure output OTA contains only 4
partitions

Change-Id: Id79e4bc4c5fb9d1225cb2aa7c1823afccc0b61f2