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/CMakeLists.txt b/boot/zephyr/CMakeLists.txt
index b660bc7..81b76f4 100644
--- a/boot/zephyr/CMakeLists.txt
+++ b/boot/zephyr/CMakeLists.txt
@@ -102,7 +102,7 @@
   ${BOOT_DIR}/bootutil/src/bootutil_misc.c
   )
 
-if(CONFIG_SINGLE_IMAGE_DFU)
+if(CONFIG_SINGLE_APPLICATION_SLOT)
 zephyr_library_sources(
   ${BOOT_DIR}/zephyr/single_loader.c
   )