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/Kconfig b/boot/zephyr/Kconfig
index 2e416b8..0cf133a 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -58,8 +58,8 @@
 
 menu "MCUBoot settings"
 
-config SINGLE_IMAGE_DFU
-	bool "Single image application"
+config SINGLE_APPLICATION_SLOT
+	bool "Single slot bootloader"
 	default n
 	help
 	  Single image area is used for application which means that
@@ -163,7 +163,7 @@
 	  every boot, but can mitigate against some changes that are
 	  able to modify the flash image itself.
 
-if !SINGLE_IMAGE_DFU
+if !SINGLE_APPLICATION_SLOT
 choice
 	prompt "Image upgrade modes"
 	default BOOT_SWAP_USING_MOVE if SOC_FAMILY_NRF
@@ -206,7 +206,7 @@
 endchoice
 endif
 
-if !SINGLE_IMAGE_DFU
+if !SINGLE_APPLICATION_SLOT
 config BOOT_BOOTSTRAP
 	bool "Bootstrap erased the primary slot from the secondary slot"
 	default n
@@ -424,7 +424,7 @@
 config UPDATEABLE_IMAGE_NUMBER
 	int "Number of updateable images"
 	default 1
-	range 1 1 if SINGLE_IMAGE_DFU
+	range 1 1 if SINGLE_APPLICATION_SLOT
 	help
 	  Enables support of multi image update.