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/include/sysflash/sysflash.h b/boot/zephyr/include/sysflash/sysflash.h
index f651779..99ca273 100644
--- a/boot/zephyr/include/sysflash/sysflash.h
+++ b/boot/zephyr/include/sysflash/sysflash.h
@@ -6,7 +6,7 @@
 #include <devicetree.h>
 #include <mcuboot_config/mcuboot_config.h>
 
-#ifndef CONFIG_SINGLE_IMAGE_DFU
+#ifndef CONFIG_SINGLE_APPLICATION_SLOT
 
 #if (MCUBOOT_IMAGE_NUMBER == 1)
 /*
@@ -43,7 +43,7 @@
 #define FLASH_AREA_IMAGE_SCRATCH    FLASH_AREA_ID(image_scratch)
 #endif
 
-#else /* CONFIG_SINGLE_IMAGE_DFU */
+#else /* CONFIG_SINGLE_APPLICATION_SLOT */
 
 #define FLASH_AREA_IMAGE_PRIMARY(x)	FLASH_AREA_ID(image_0)
 #define FLASH_AREA_IMAGE_SECONDARY(x)	FLASH_AREA_ID(image_0)
@@ -53,6 +53,6 @@
  */
 #define FLASH_AREA_IMAGE_SCRATCH	0
 
-#endif /* CONFIG_SINGLE_IMAGE_DFU */
+#endif /* CONFIG_SINGLE_APPLICATION_SLOT */
 
 #endif /* __SYSFLASH_H__ */