Allow an optional devtype parameter to the ums command, which specifies
the type of the device to be exported. This could allow exporting a SATA
or even another USB device.

Cc: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>