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.h b/payload_consumer/payload_constants.h
index 43c3137..0833484 100644
--- a/payload_consumer/payload_constants.h
+++ b/payload_consumer/payload_constants.h
@@ -63,8 +63,8 @@
 // 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 kPartitionNameKernel[];
+extern const char kPartitionNameRoot[];
 
 extern const char kBspatchPath[];
 extern const char kDeltaMagic[4];