usb: gadget: f_sdp: Provide filesize env variable for downloaded images

usb: gadget: f_sdp: Provide filesize env variable for downloaded images

Currently it's not possible to get filesize of downloaded images and
it's impossible to automate some tasks in scripts. So this patch adds
`filesize` environment variable with size (as hex number in bytes) of
the last successfully downloaded file via `sdp` command.

Cc: Lukasz Majewski <lukma@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Reviewed-by: Stefan Agner <stefan.agner@toradex.com>