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.

(From OE-Core rev: 4f8ed1b3bf676a58055ebe01184b3594459a4118)

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>