all: sync and dedup the creation of the SYSNAME and OBJTYPE env vars (#321) This should prevent the issues of dist/buildmk and src/mkhdr getting out of synchronization yet again. I also add a rule for arm64 to the OBJTYPE sed command. Fixes #243 Fixes #320 Change-Id: I60f69a1f32b5ed5ae5ac8a1659c38e29debed005