show_config: add DISTRO/show_config as function source and use it

Signed-off-by: Ian Leonard <antonlacon@gmail.com>

config/show_config

@@ -105,7 +105,13 @@ show_config() {
   done
 
   # Distribution specific configuration
+  # show_distro_config() should be included in one of:
+  #   $DISTRO/config/functions
+  #   $DISTRO/show_config
 
+  if [ -f distributions/${DISTRO}/show_config ]; then
+    . distributions/${DISTRO}/show_config
+  fi
   if [ "$(type -t show_distro_config)" = "function" ]; then
     show_distro_config
   fi