Add an updater function to compute hash tree

The new command is part of the transfer.list and allows us to compute the hash
tree on non-ab devices.

The required arguments for the hash_tree computation are:
  hash_tree_ranges
  source_ranges
  hash_algorithm
  salt_hex
  root_hash

Bug: 25170618
Test: unit tests pass;  run simulator with compute_hash_tree
Change-Id: I8ff0d582cc8adabb8a060db7845f38b35b28e62c
diff --git a/updater/include/private/commands.h b/updater/include/private/commands.h
index 087d7cf..7f9dc79 100644
--- a/updater/include/private/commands.h
+++ b/updater/include/private/commands.h
@@ -213,6 +213,10 @@
 //      - Free the given stash data.
 //      - Meaningful args: StashInfo
 //
+//    compute_hash_tree <hash_tree_ranges> <source_ranges> <hash_algorithm> <salt_hex> <root_hash>
+//      - Computes the hash_tree bytes and writes the result to the specified range on the
+//        block_device.
+//
 //    abort
 //      - Abort the current update. Allowed for testing code only.
 //
@@ -221,6 +225,7 @@
   enum class Type {
     ABORT,
     BSDIFF,
+    COMPUTE_HASH_TREE,
     ERASE,
     FREE,
     IMGDIFF,