Split make_system_path_relative_to_its_root into separate file

relpath.c from getroot.c as it's common between unix and haiku
	but otherwise haiku doesn't use any functions from unix getroot.c.

grub-core/osdep/aros/relpath.c

@@ -0,0 +1,75 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2013  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <config-util.h>
+#include <config.h>
+
+#include <grub/util/misc.h>
+
+#include <grub/mm.h>
+#include <grub/misc.h>
+#include <grub/emu/misc.h>
+#include <grub/emu/hostdisk.h>
+#include <grub/emu/getroot.h>
+
+#include <string.h>
+#include <dos/dos.h>
+#include <dos/filesystem.h>
+#include <dos/exall.h>
+#include <proto/dos.h>
+#include <proto/exec.h>
+#include <devices/trackdisk.h>
+
+char *
+grub_make_system_path_relative_to_its_root (const char *path)
+{
+  char *p;
+  unsigned char *tmp;
+  char *ret;
+  BPTR lck;
+
+  if (path[0] == '/' && path[1] == '/' && path[2] == ':')
+    return xstrdup (path);
+
+  tmp = xmalloc (2048);
+
+  lck = Lock ((const unsigned char *) path, SHARED_LOCK);
+  if (!lck || !NameFromLock (lck, tmp, 2040))
+    {
+      free (tmp);
+      tmp = (unsigned char *) xstrdup (path);
+    }
+  if (lck)
+    UnLock (lck);
+  p = strchr ((char *) tmp, ':');
+  if (!p)
+    return (char *) tmp;
+  if (p[1] == '/' || p[1] == '\0')
+    {
+      ret = xstrdup (p + 1);
+    }
+  else
+    {
+      ret = xmalloc (strlen (p + 1) + 2);
+      ret[0] = '/';
+      strcpy (ret + 1, p + 1);
+    }
+
+  free (tmp);
+  return ret;
+}