examples: Use a common serial_printf()

As the printf() function has not been retargeted to use MCU-Driver-HAL's
serial, we have a custom function in each example's main.c for printing
to serial. To avoid duplicating this function in every example which
makes it hard to maintain, move it into a new library and name the
function serial_printf().

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>