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>
diff --git a/boot/boot_serial/src/boot_serial.c b/boot/boot_serial/src/boot_serial.c
index 841a4f2..a387bdc 100644
--- a/boot/boot_serial/src/boot_serial.c
+++ b/boot/boot_serial/src/boot_serial.c
@@ -63,12 +63,8 @@
 
 BOOT_LOG_MODULE_DECLARE(mcuboot);
 
-#ifndef BOOT_IMAGE_NUMBER
-#define BOOT_IMAGE_NUMBER MCUBOOT_IMAGE_NUMBER
-#endif
-
 #define BOOT_SERIAL_INPUT_MAX   512
-#define BOOT_SERIAL_OUT_MAX     (128 * MCUBOOT_IMAGE_NUMBER)
+#define BOOT_SERIAL_OUT_MAX     (128 * BOOT_IMAGE_NUMBER)
 
 #ifdef __ZEPHYR__
 /* base64 lib encodes data to null-terminated string */