trace: fix 'trace.py download'

Broken by 3730c3953ca0cbae940f41134f73764ec1abb9d1.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
trace: make 'trace.py download' convert the buffers to tracefile by default

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>