`led_init()` is already protected by an `#ifdef`, so no need for
the weak symbol.
The problem is that the weak function does not get overridden, even when
the board provides it's own led_init(), resulting in no LEDs being
initialized.
Moving atmega_stdio_init() to cpu_init() just before periph_init() guarantees
that stdio is available to allow DEBUG() in periph_init(). This also helps to
unify the boot up process of ATmega boards and de-duplicates the stdio init from
board_init().
