cmake: KCONFIG_ROOT does not need to be set explicitly

The presence of a Kconfig file will be checked by the build system so
it is not necessary to explicitly set KCONFIG_ROOOT.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
diff --git a/boot/zephyr/CMakeLists.txt b/boot/zephyr/CMakeLists.txt
index 12dd42b..e31e531 100644
--- a/boot/zephyr/CMakeLists.txt
+++ b/boot/zephyr/CMakeLists.txt
@@ -6,8 +6,6 @@
 
 cmake_minimum_required(VERSION 3.8.2)
 
-set(KCONFIG_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/Kconfig)
-
 # Board-specific CONF_FILES should get merged into the build as well.
 #
 # Do this by defining the set_conf_file macro: