delta_generator: Use "kernel" and "root" partition names internally

This commit abandons using "boot" and "system" names internally for
major version 1 payloads in favor of "kernel" and "root", which is
symmetrical with what's used in major version 2 payloads.

BUG=chromium:794404
TEST=no errors running brillo_update_payload with FORCE_MAJOR_VERSION
  set to either 1 or 2; paycheck.py <generated payload> --check and
  printing partition names in protobuf confirms correct names are
  generated

Change-Id: I396274812d5586e19a021c05b8b7a415aac976bf
Reviewed-on: https://chromium-review.googlesource.com/1119573
Commit-Ready: Tudor Brindus <tbrindus@chromium.org>
Tested-by: Tudor Brindus <tbrindus@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/payload_consumer/payload_constants.cc b/payload_consumer/payload_constants.cc
index 797e76d..6e7cd00 100644
--- a/payload_consumer/payload_constants.cc
+++ b/payload_consumer/payload_constants.cc
@@ -36,8 +36,8 @@
 
 const uint64_t kMaxPayloadHeaderSize = 24;
 
-const char kLegacyPartitionNameKernel[] = "boot";
-const char kLegacyPartitionNameRoot[] = "system";
+const char kPartitionNameKernel[] = "kernel";
+const char kPartitionNameRoot[] = "root";
 
 const char kDeltaMagic[4] = {'C', 'r', 'A', 'U'};