configs: stm32f769-disco: Fix console cmdline

The Linux cmdline encoded in the defconfig is wrong, the
STM32 USART driver registers as ttySTM0 not ttyS0.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
1 file changed