*-ssl variants of mosquitto require libopenssl, which increases the
storage requirement by almost 1 MB, even when compressed with squashfs.
Because we currently do not need TLS support for fff-mqtt, switch to the
nossl variant to save space and allow building for devices with 8 MiB
flash.
Signed-off-by: Fabian Bläse <fabian@blaese.de>