boot: zephyr: Use private key

Change adds using own private key for MCUBoot bootloader.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
diff --git a/boot/zephyr/CMakeLists.txt b/boot/zephyr/CMakeLists.txt
index 6cdb024..abe931e 100644
--- a/boot/zephyr/CMakeLists.txt
+++ b/boot/zephyr/CMakeLists.txt
@@ -231,9 +231,14 @@
     )
 endif()
 
+# CONF_FILE points to the KConfig configuration file of the bootloader
+get_filename_component(CONF_DIR ${CONF_FILE} DIRECTORY)
+
 if(NOT CONFIG_BOOT_SIGNATURE_KEY_FILE STREQUAL "")
   if(IS_ABSOLUTE ${CONFIG_BOOT_SIGNATURE_KEY_FILE})
     set(KEY_FILE ${CONFIG_BOOT_SIGNATURE_KEY_FILE})
+  elseif(EXISTS ${CONF_DIR}/${CONFIG_BOOT_SIGNATURE_KEY_FILE})
+    set(KEY_FILE ${CONF_DIR}/${CONFIG_BOOT_SIGNATURE_KEY_FILE})
   else()
     set(KEY_FILE ${MCUBOOT_DIR}/${CONFIG_BOOT_SIGNATURE_KEY_FILE})
   endif()