Skip hashtree computation during block image verify

The hashtree computation is designed to execute after we write all the
bytes to the target block device. And executing the command during block
image verify will almost always fail since we are still on the source
build.

Test: run simulator
Change-Id: If8ebb66739969520367a0815f5f8f89f6fae47cf

updater/blockimg.cpp

@@ -2018,7 +2018,7 @@ Value* BlockImageVerifyFn(const char* name, State* state,
     // clang-format off
     { Command::Type::ABORT,             PerformCommandAbort },
     { Command::Type::BSDIFF,            PerformCommandDiff },
-    { Command::Type::COMPUTE_HASH_TREE, PerformCommandComputeHashTree },
+    { Command::Type::COMPUTE_HASH_TREE, nullptr },
     { Command::Type::ERASE,             nullptr },
     { Command::Type::FREE,              PerformCommandFree },
     { Command::Type::IMGDIFF,           PerformCommandDiff },