[RESTORED!] 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: I3b5a14c530ff4b10d97f67636237d96875eb8969
Author: Michael Sevakis

uisimulator/common/filesystem-sim.c

@@ -309,6 +309,8 @@ int sim_get_os_path(char *buffer, const char *path, size_t bufsize)
 
             const char *next;
             volume = path_strip_volume(p, &next, true);
+            if (volume == ROOT_VOLUME)
+                volume = 0; /* FIXME: root no longer implies volume 0 */
 
             if (next > p)
             {