lib: remove unnecessary asserts in bsearch
platform: use openocd to flash stm32f4 as fallback method
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>
bump copyright date for banner
revise the license terms
README: update verified toolchain list
improve the descriptions about external sources