boards: stm32f4_disco: Fix console documentation

Unlinke nucleo or disco boards, ST-Link Tx/Rx pins used for Virtual
Com Port is not connected to the chip.
Then, console is not available by default and one should use a serial
cable to enjoy console. Adding mention of this fact to the board
documentation and replacing default hello_world example by blinky,
since blinky sample works without any additional hardware.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>