Support UART for SDP

Vybrid SoC supports the serial download protocol (SDP) over UART. This changes split the protocol part (imx_sdp.c) and the USB protocol stack (imx_usb.c). The new file (imx_uart.c) adds support for serial port. 

This patch was tested on Vybrid over UART and USB as well as on a i.MX 6 device over USB.