avr32: Use board_postclk_init instead of gclk_init

avr32: Use board_postclk_init instead of gclk_init

Replace the avr32-specific gclk_init() board hook with the standard
board_postclk_init() hook which is supposed to run at the same point
during initialization.

Provide a dummy weak alias for boards not implementing this hook. The
cost of this is:
  - 2 bytes for the dummy function (retal 0)
  - 2 bytes for each unnecessary function call (short rcall)

which is a pretty small price to pay for avoiding lots of #ifdef
clutter. In this particular case, all boards probably end up slightly
smaller because we avoid the conditional checking if the gclk_init
symbol is NULL.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>