luci-app-attendedsysupgrade: activate in buildbot

luci-app-attendedsysupgrade: activate in buildbot

Instead of making the main LuCI packages depend on
luci-app-attendedsysupgrade, only activate it by default when BUILDBOT
is seletced.

This way it is not selected by default in custom builds, but it will be
automatically selected in builds with CONFIG_BUILDBOT set. Even when
CONFIG_BUILDBOT is set it is possible to deactivate it.

This still depends on LuCI, it will only be activated in OpenWrt release
builds which are build with LuCI and not in OpenWrt main branch builds.

Signed-off-by: Hauke Mehrtens <redacted>