package/systemd: group all journal-remote tools under a single option

The -Dremote=true flag enables three tools: systemd-journal-gatewayd,
systemd-journal-remote and systemd-journal-upload.

It is not possible with simple means to install them seperately. So use
the systemd-journal-remote option to enable or disable them all together.

Drop systemd-journal-gatewayd option and add it to legacy.

Fixes: #12301

Signed-off-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>