ums: support multiple LUNs at once

Extend the ums command to accept a list of block devices. Each of these
will be exported as a separate LUN. An example use-case would be:

ums 0 mmc 0,0.1,0.2

... which would export LUNs for eMMC 0's user data, boot0, and boot1 HW
partitions. This is useful since it allows the host access to everything
on the eMMC without having to somehow stop the ums command from executing
and restart it with different parameters.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Tom Rini <trini@konsulko.com>