screendump.c: use HOME_DIR instead of the hard-coded "" (root)

This adds the support for hosted or other
particular platforms, where the "root" ("home")
path is NOT "/" (or "").
The change affects screendump in particular.

Change-Id: I4c218d102fe12cc782aedb99d456e9f72679ea07
Reviewed-on: http://gerrit.rockbox.org/613
Reviewed-by: Thomas Martitz <kugel@rockbox.org>
diff --git a/firmware/screendump.c b/firmware/screendump.c
index 0dfe79a..28c3761 100644
--- a/firmware/screendump.c
+++ b/firmware/screendump.c
@@ -21,6 +21,7 @@
 
 #include "config.h"
 #include "screendump.h"
+#include "rbpaths.h"
 
 #include "file.h"
 #include "general.h"
@@ -120,9 +121,9 @@
 #endif
 
 #if CONFIG_RTC
-    create_datetime_filename(filename, "", "dump ", ".bmp", false);
+    create_datetime_filename(filename, HOME_DIR, "dump ", ".bmp", false);
 #else
-    create_numbered_filename(filename, "", "dump_", ".bmp", 4
+    create_numbered_filename(filename, HOME_DIR, "dump_", ".bmp", 4
                              IF_CNFN_NUM_(, NULL));
 #endif