[gpio] move gpio_setup_x declarations to implementation

properly document that you can actually set multiple gpios at a time
[lpc21] implement gpio functions
[lcp21] fix gpio_toggle
[datalink] w5100: arch indep gpio functions for drdy pin
Merge pull request #651 from paparazzi/gpio

Gpio interface improvements

Common gpio functions for lpc21 and stm32 architectures like:
- gpio_setup(GPIOA, GPIO1)
- gpio_clear(GPIOA, GPIO1)
- gpio_setup_output(GPIOA, GPIO1)
- gpio_setup_input(GPIOA, GPIO1)

you can also set multiple gpios at once, e.g.: `gpio_set(GPIOA, GPIO1 | GPIO2)`