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>
1 file changed