Enable Serial Download Protocol (SDP) in SPL and U-Boot. This is
useful to make use of imx_usb to download the complete U-Boot
(u-boot.img) after SPL has been downloaded. The U-Boot command
sdp allows to enumerate as SDP capable device again, e.g. to
download a Linux kernel and/or U-Boot script.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>