dfu: add support for the dfu_alt_info reintialization from the flashed script

Reinitialize DFU USB gadget after flashing the 'SCRIPT' entity to ensure
that the potential changes to the 'dfu_alt_info' environment variable are
applied.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
[lukma - I've moved the bool retry to avoid build (CI) errors]