- cpu/stm32f1: Removed previous code in gpio_init() to provide PB4 on the
Nucleo-F103RB only
- cpu/stm32_common: Introduced STM32F1_DISABLE_JTAG which, if defined in
board.h, exposes the JTAG only pins as GPIOs. This keeps the SWD pins, so that
SWD debugging remains possible