[sample] gpio input with button
stm32f4: fix incorrect baud rate calculation for USART2

USART1 and USART6 are on APB2 whose frequency is 84MHz,
while USART2, USART3, UART4, and UART5 are on APB1 whose
frequency is 42
MHz (max).

Commented-by: Shih-Po Hung <arcbbb@gmail.com>
Signed-off-by: Jim Huang <jserv@0xlab.org>
[gpio] sample with two threads one for led control and the other for button input probe
Merge remote-tracking branch 'upstream/master'
Merge branch 'develop' into feature/button_input
set to default sample
Merge branch 'feature/button_input' into develop