Move install operations to partitions field in major version 2.

install_operations and kernel_install_operations will be empty in major
version 2, they are now present in partitions field.

partition_info are still in old location, more refactoring are needed before
moving them.

This patch also moves the kLegancyPartitionName constants from install_plan.h
to payload_constants.h.

TEST=cros_generate_update_payload generate same payload version 1 as before.
Call delta_generator manually with --major_version=2, then cros payload show.
cros_workon_make update_engine --test

Bug: 23694580
Change-Id: If9e9531656480b1174e13b39af7ecb6a81060aac
diff --git a/payload_constants.h b/payload_constants.h
index 55a82d3..188ea84 100644
--- a/payload_constants.h
+++ b/payload_constants.h
@@ -23,6 +23,12 @@
 
 namespace chromeos_update_engine {
 
+// The kernel and rootfs partition names used by the BootControlInterface when
+// handling update payloads with a major version 1. The names of the updated
+// partitions are include in the payload itself for major version 2.
+extern const char kLegacyPartitionNameKernel[];
+extern const char kLegacyPartitionNameRoot[];
+
 extern const char kBspatchPath[];
 extern const char kDeltaMagic[];