SPL: Add YMODEM over UART load support

Adds support for loading U-Boot from UART using YMODEM protocol.
If YMODEM support is enabled in SPL and the romcode indicates
that SPL loaded via UART then SPL will wait for start of a
YMODEM transfer via the console port.

Signed-off-by: Matt Porter <mporter@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>