cmd: ums: add getting the default parameters from env

cmd: ums: add getting the default parameters from env

Add reading command parameters from the environment. The same environment
variables are used as for the "dfu" and "thor" commands. This allows to
specify USB controller ("dfu_usb_con"), device interface ("dfu_interface")
and device number ("dfu_device"). Such change allows to call "ums" without
any parameters for the default storage device.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: If725408159937dc50cb375d5c80455dac5cf9b37