[nrf fromlist] boot_serial: Increase BOOT_SERIAL_OUT_MAX

Upstream PR:
mcu-tools/mcuboot#1144

Change increases BOOT_SERIAL_OUT_MAX. This is necessary to provide
complete image list when multi-image DFU is enabled and four image
slots are occupied.

(cherry picked from commit 0a49964c476974a77176f6bd8b935950a783d078)
Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>

boot/boot_serial/src/boot_serial.c

@@ -64,7 +64,7 @@
 MCUBOOT_LOG_MODULE_DECLARE(mcuboot);
 
 #define BOOT_SERIAL_INPUT_MAX   512
-#define BOOT_SERIAL_OUT_MAX     128
+#define BOOT_SERIAL_OUT_MAX     (128 * MCUBOOT_IMAGE_NUMBER)
 
 #ifdef __ZEPHYR__
 /* base64 lib encodes data to null-terminated string */