ucm: implement ValueDefaults.BootCardGroup and define use

ucm: implement ValueDefaults.BootCardGroup and define use

We need a boot synchronization for multiple UCM cards where linking
is expected like AMD ACP or Intel AVS drivers. This method is
using a timestamp file which can be created and modified during
the boot process (e.g. from the alsactl tool).

The goal is to return a valid UCM configuration for standard
applications combining multiple ALSA cards into one UCM configuration
and cover the time window when all cards have not been probed yet.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>