Generate the compute_hash_tree command

Generate the transfer command "compute_hash_tree" for incremental
updates of the non-A/B devices that enable verified boot 1.0

Other changes include:
i.  factor out verity_utils to use both in build_image and blockimgdiff
ii. add a new flag 'hashtree_info_generator' in sparse_image to generate
    the hashtree information.

Bug: 25170618
Test: generate a package for aosp_angler; and run simulator
Change-Id: I4d4d7a3e41dc3da810d2cbf8988e85d187c9ab0e