libcutils: handle root directory empty path

Usually, the canned paths lack the leading '/' which means that the
root directory is an empty path.  This patch makes
load_canned_fs_config() handle this empty path as the root directory.
It also make it flexible enough to handle an optional leading '/'.

Original-Author: Jeremy Compostella <jeremy.compostella@intel.com>
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Signed-off-by: Gaelle Nassiet <gaellex.nassiet@intel.com>
Change-Id: Ice759ef37d1df23a54df1158ec0d3ad7f577a069