cmd: usbdown: support the usbdown from usb storage to mmc

cmd: usbdown: support the usbdown from usb storage to mmc

This command is to support the download mode from usb storage to mmc
block device. XU4 didn't support the thor download, so this solution
might be replaced to thor download.

* How to use this command
1. Assume that  Block number of eMMC block device is "0"
2. If you want to flash the rootfs.img,
	$update 0:0 mmc rootfs.img
	- 0:0 -> usb block 0 devices and partitioin 0
	  if you just enter "0" instead of "0:0", that check the
	  partition 1 by default.
	- mmc -> interface to flash the image
	- rootfs.img -> filename to flash (directory is "/" by default.)
	  If you want to use the directory, can use likes "/tizen/rootfs.img".
	  (There is rootfs.img under "tizen" directory.)
3. Use "update all" command
	$usbdown all
	- directory is "/updateTizen/ by default.
*Recommendation
- Use "usbdown all"

Change-Id: I267c9b871741e9e1f6aef29965f1f846e19864f1
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>