payloads/filo: Set stable tag to something that builds

payloads/filo: Set stable tag to something that builds

Also rename the prompt to "tested" to make it more obvious that there
is no really stable version.

Change-Id: Ib719fe5c30783a53ddad2a2dc2d9ecda37a05ac2
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46849
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>