tagcache: add a setting for customizing the database path

Add a new setting,

    database path: /path/to/folder

to change where the database files are stored, which allows it to
be shared by multiple builds when using multiboot. This avoids the
need to maintain a separate copy of the database for each build.
This setting can only be set from the config file; it has no menu
option yet (due to lack of a GUI to pick the directory).

Change-Id: Ide7b3ccdd84abb62b52f900421bd3d101773e093

apps/settings_list.c

@@ -1544,6 +1544,8 @@ const struct settings_list settings[] = {
                   "gather runtime data", NULL),
     TEXT_SETTING(0, tagcache_scan_paths, "database scan paths",
                  DEFAULT_TAGCACHE_SCAN_PATHS, NULL, NULL),
+    TEXT_SETTING(0, tagcache_db_path, "database path",
+                 ROCKBOX_DIR, NULL, NULL),
 #endif
 
     /* replay gain */