avr32: set CONFIG_CC_OPTIMIZE_FOR_SIZE=y for all defconfigs

This patch makes sure the kconfig option CC_OPTIMIZE_FOR_SIZE is set to yes for
all default configuration files. This ensures the kernel is optimized for size,
and avoids potential relocation truncated to fit problems.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>