Implement the detection of USB Flash devices on IVI.

BUG=XWALK-2040

This patch implement the feature of detecting USB Flash devices on ivi, as well as getting its "capacity" and "isRemovable" attributes.
About the ``capability`` attribute, there are no official doc describing this. I made some tests in terminal by running ``$ udevadm info --attribute-walk --name=/dev/sdc ``, which would show all storage devices' attributes supported by udev, such as ATTR{size}, ATTR{removable} and ATTR{capability}. The capability represent whether the storage is a disk device, a sd-card or a u-flash. 
And I think I need more time to get desktop and Tizen unified and learn how to use the udev event to replace timeout.