{release,dev}build: clarify signaling variables
{release,dev}build: clarify signaling variables

Clarify all variables before doing any bulk operation within `devbuild` or
`releasebuild` to avoid any confusion (e.g. `DEVELOPER_BUILD_RUNNING` and
`DEVELOPER_BUILD_SUCCEEDED` are set at the same time).

Signed-off-by: Jens Rehsack <69d49c504a9219a55b8be0f905e8fe3c32ffb57f@netbsd.org>

rehsack/meta-jens,rehsack/meta-jens,rehsack/meta-jens