stm32f4: add serial print port

Add the stm32F4 board's serial ports support.
User can use it easily.
The user only need to edit the number of the usart.
The patch also fix the serial print out.

Last, this version of patch fix the first patch checkpatch.pl error.
Thanks to Kamil Lulko.

Signed-off-by: kunhuahuang <huangkunhua@gmail.com>