Enhance the UMS test to optionally mount a partition and read/write a file
to it, validating that the content written and read back are identical.

This enhancement is backwards-compatible; old boardenv contents that don't
define the new configuration data will cause the test code to perform as
before.

test/ums/ is deleted since the Python test now performs the same testing
that it did.

The code is also re-written to make use of the recently added utility
module, and split it up into nested functions so the overall logic of
the test process can be followed more easily without the details
cluttering the code.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>

This change helps to run script on machines with quite long uptime.
Without this the following error emerges:

File: ./dat_14M.img
umount: /mnt/tmp-ums-test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
TX: md5sum:083d3d22b542d3ecba61b12d17e03f9f
mount: /dev/sdd6 already mounted or /mnt/tmp-ums-test busy
mount: according to mtab, /dev/sdd6 is already mounted on /mnt/tmp-ums-test

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>

This commit adds new test for UMS USB gadget to u-boot mainline tree.
It is similar in operation to the one already available in test/dfu
directory.

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