boot/zephyr: renamed single-image mode to single-application-slot mode

The terms "single-image boot" and the "single image application"
of Zephyr are confused. It might be might be understood as one-pair image
dual-bank mode in comparison to multi-image mode.

This patch replaces CONFIG_SINGLE_IMAGE_DFU by
CONFIG_SINGLE_APPLICATION_SLOT name,
which makes this mode name unambiguous.

fixes #800

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

fix
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index ffa4500..50141f5 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -57,7 +57,7 @@
 {
     switch (slot) {
     case 0: return FLASH_AREA_IMAGE_PRIMARY(image_index);
-#if !defined(CONFIG_SINGLE_IMAGE_DFU)
+#if !defined(CONFIG_SINGLE_APPLICATION_SLOT)
     case 1: return FLASH_AREA_IMAGE_SECONDARY(image_index);
 #if !defined(CONFIG_BOOT_SWAP_USING_MOVE)
     case 2: return FLASH_AREA_IMAGE_SCRATCH;
@@ -78,7 +78,7 @@
     if (area_id == FLASH_AREA_IMAGE_PRIMARY(image_index)) {
         return 0;
     }
-#if !defined(CONFIG_SINGLE_IMAGE_DFU)
+#if !defined(CONFIG_SINGLE_APPLICATION_SLOT)
     if (area_id == FLASH_AREA_IMAGE_SECONDARY(image_index)) {
         return 1;
     }