Set console, passed to kernel by default, do ttyS2, which corresponds
to UART3. Unfortunatelly, there's no way to use UART1 (and ttyS0) on
this board, just because the GPIO pins used for RS-232 here in alter-
native functions have only UART3 and UART4 capabilities (btw, I tested
that being configured to use UART4 - U-Boot console works well too).

Also, fix-up platform configuration, and TFTP boot dir (in linux we'll
assume STM32 platform, not STM32F2 as we currently do in U-Boot).

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>