Allow mounting of any directory as the root directory.

Provide definitions for the macros:
* RB_ROOT_VOL_HIDDEN(v) to exclude certain items from the root.
* RB_ROOT_CONTENTS to return a string with the name of the
directory to mount in the root.

Defaults are in export/rbpaths.h

It's a bit much for those that don't need the full functionality.
Some conditional define can cut it back a lot to cut out things only
needed if alternate root mounts are required. I'm just not bothering
yet. The basic concept would be applied to all targets to keep file
code from forking too much.

Change-Id: I90b5c0a1c949283d3102c16734b0b6ac73901a30

firmware/export/rbpaths.h

@@ -64,6 +64,9 @@
 #define PLUGIN_DIR          ROCKBOX_DIR "/rocks"
 #define CODECS_DIR          ROCKBOX_DIR "/codecs"
 
+#define RB_ROOT_VOL_HIDDEN(v)   (IF_MV_VOL(v) == 0)
+#define RB_ROOT_CONTENTS_DIR    "/" IF_MV("<0>")
+
 #else /* APPLICATION */
 
 #define HOME_DIR "<HOME>" /* replaced at runtime */