usb: gadget: sdp: extend images compatible for jumps

Support U-Boot images in SPL so that u-boot.img files can be
directly downloaded and executed. Furthermore support U-Boot
scripts download and execution in full U-Boot so that custom
recovery actions can be downloaded from the host in a third
step.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>