README: add verified version of GNU Tools for ARM Embedded
Documentation: tweak coding style slightly
make USART pin assignment configurable

Fixes #62

In preparation of new boards support other than STM32F4 Discovery, it is
necessary to make the USART pin assignment configurable. At present, both
USART4 (default) and USART2 of STM32F4DISCOVERY are supported depending on
Kconfig results.