boot_serial: cleanup max output size calculation

Calculation of BOOT_SERIAL_OUT_MAX was based on MCUBOOT_IMAGE_NUMBER
while in other places BOOT_IMAGE_NUMBER is taken into account as
number of images. Let's align the calculation to use same literal value
as others.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>