arm/frdm_k64f: fix boot issue with MBED bootloader and INIT_STACKS=y

Initializing the interrupt stack before initializing (turning off) the
watchdog on the FRDM board pushed the initialization of the watchdog too
late, causing it to fire and reset the board. The board would be kept in
a reboot loop.

Move the initialization of the watchdog earlier: this runs on the main
stack now, instead of the interrupt stack, the same stack the interrupt
stack initalization code runs on.

Change-Id: Ic0006f4f4f4090393571d8355a80dc9390c9fbc6
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>