tcf-agent: kill with USR2 in systemd stop

tcf-agent: kill with USR2 in systemd stop

tcf-agent ignores SIGTERM, so upstream uses USR2 instead. This issue was noticed
by Jan Kiszka and Brian Avery around the same time:

https://patchwork.openembedded.org/patch/139546/
https://patchwork.openembedded.org/patch/139560/

However, these patches fixed only the init scripts, not the systemd service
file. This patch fixes the systemd file.

Change-Id: Id59fab32ae78213efe98a646dce2eec0881720f6
Signed-off-by: Martin Kelly <mkelly@xevo.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9597
Reviewed-by: José Bollo <jobol@nonadev.net>
Reviewed-by: Changhyeok Bae <changhyeok.bae@gmail.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>