conf: Allow dynamic top-level config directory

Currently the top-level config directory is specified only via
configure script option, and is fixed after that.  It's inconvenient
when the library is moved to another base directory, or if you want to
use a library code (e.g. with $LD_PRELOAD) with the incompatible
config setups.

This patch allows user to override the top-level config path via the
environment varialbe, $ALSA_CONFIG_DIR.  For that, a new helper
function, snd_config_topdir(), was introduced, and the codes referring
to the top config dir have been modified to handle it dynamically.

Signed-off-by: Takashi Iwai <tiwai@suse.de>