test/py: dfu: allow boardenv to specify test sizes

test/py: dfu: allow boardenv to specify test sizes

Allow the env__dfu_configs boardenv data to specify the set of DFU
transfer sizes to test. Manually specifying test sizes is useful if you
wish to test multiple DFU configurations (e.g. SD card ext4 filesystem, SD
card whole raw partition, RAM, etc.), but don't want to test every
single transfer size on each, to avoid bloating the overall time taken by
testing. If the boardenv doesn't specify a set of sizes, the built-in list
is used as a default, preserving backwards-compatibility.

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