boot/zephyr: make flash_area_erased_val() weak
boot/zephyr: make flash_area_erased_val() weak

The function was made week so zephyr-rtos implementation
will be used if available.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot