Add binary to write basic COW

Adding small binary to write a simple COW for version 2. We will then
use updated reader to make sure we can read this version 2 cow. Think it
would be a good idea to keep the binary here to see how exactly we
generated the cow (useful for debugging and if we ever need to recreate
this cow)

Test: m basic_v2_cow_writer
Change-Id: I28435025b7a8280fc5c4574876cc9110b391cb0e