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.cc b/payload_constants.cc
index 05f114d..9d6fb3d 100644
--- a/payload_constants.cc
+++ b/payload_constants.cc
@@ -18,6 +18,9 @@
 
 namespace chromeos_update_engine {
 
+const char kLegacyPartitionNameKernel[] = "boot";
+const char kLegacyPartitionNameRoot[] = "system";
+
 const char kDeltaMagic[] = "CrAU";
 const char kBspatchPath[] = "bspatch";