test: dfu: Extend dfu_gadget_test_init.sh to accept sizes of test files

It is now possible to pass to the dfu_gadget_test_init.sh script the sizes
of files to be generated.

This feature is required by UMS tests which reuse this code.

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