[nelson][paver] Support bl2+tpl combined image

Add support for flashing combined bl2+tpl image for nelson.
Specifically, it flashes bl2+tpl to boot0/1 and then extracts tpl part
and flashes to either tpl_a or tpl_b depending on the given
configuration.

This is achieved by introducing a nelson bootloader specific partition
client that forms as a composite of PartitionCopyClient and
FixedOffsetBlockPartitionClient. One tricky issue is how to define the
behavior of read. Becuase tpl_r in boot0/1 will likely be different from
tpl_a/b as we keep performing A/B update. For now, the read method will
first compare tpl_r with tpl_a/b and only proceed to read bl2+tpl if
they are the same. Otherwise, it errors out.

Change-Id: I91d1895e6f7e24be330ea2c1260bfa8c24cb7fc1
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/448056
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
Reviewed-by: David Pursell <dpursell@google.com>
Reviewed-by: Suraj Malhotra <surajmalhotra@google.com>
Testability-Review: David Pursell <dpursell@google.com>