Fix for alsa-lib cross-compilation problems with ALSA_CONFIG_DIR and ALSA_PLUGIN_DIR

Fix for alsa-lib cross-compilation problems with ALSA_CONFIG_DIR and ALSA_PLUGIN_DIR

"./configure" options for selecting ALSA configuration (default
/usr/share/alsa) and plugin (/usr/lib/alsa-lib) directories introduced
by alsa-hg/alsa-lib changeset 2284 cause problems with cross-compilation
and packaging - there is no way to redefine them in runtime, during
installation phase.

This patch adds a level of indirection between constants and their
usage - alsaconfigdir for ALSA_CONFIG_DIR and alsaplugindir for
ALSA_PLUGIN_DIR - which can be redefined during "make install" stage.

Signed-off-by: Pawel MOLL <pawel.moll@st.com>