Move the commands map parsing out of PerformBlockImageUpdate(), as this
can be done more easily by the caller.
The goal (not done in this CL) is to decouple command parsing logic from
the performers. This allows (a) focusing on the command logic in the
performer; and (b) extending BBOTA commands syntax separately.
Test: Run recovery_unit_test and recovery_component_test.
Change-Id: Ife202398a7660b152d84a3ba17b90f93d19c55f2
