zephyr: Fix support for single application with serial recovery

The commit fixes compilation error when attempting to build
support for single application slot with serial recovery enabled.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/boot_serial/src/boot_serial.c b/boot/boot_serial/src/boot_serial.c
index 6325179..914167e 100644
--- a/boot/boot_serial/src/boot_serial.c
+++ b/boot/boot_serial/src/boot_serial.c
@@ -63,6 +63,10 @@
 
 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)
 
@@ -77,10 +81,6 @@
 #define htons(x) sys_cpu_to_be16(x)
 #endif
 
-#ifndef BOOT_IMAGE_NUMBER
-#define BOOT_IMAGE_NUMBER MCUBOOT_IMAGE_NUMBER
-#endif
-
 #if (BOOT_IMAGE_NUMBER > 1)
 #define IMAGES_ITER(x) for ((x) = 0; (x) < BOOT_IMAGE_NUMBER; ++(x))
 #else