MX31ADS network and flash updates

This patch depends on the previous two patches, introducing the
CONFIG_FLASH_SPANSION_S29WS_N and CONFIG_ARP_TIMEOUT configuration
options. It allows U-Boot to use buffered writes to the Spansion NOR flash
installed on this board, and eliminates long delays in network transfers
after the board startup.

Also modify flash layout to embed main and redundant environment blocks in
the U-Boot image.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>