zephyr: fix compilation when no CONFIG_BOOT_ENCRYPTION_KEY_FILE

Fix compilation of zephyr builds when there is no
CONFIG_BOOT_ENCRYPTION_KEY_FILE set.
bug introduced by #1255

Signed-off-by: Wouter Cappelle <wouter.cappelle@crodeon.com>

Author: Wouter Cappelle

boot/zephyr/CMakeLists.txt

@@ -305,22 +305,21 @@ if(NOT CONFIG_BOOT_SIGNATURE_KEY_FILE STREQUAL "")
   zephyr_library_sources(${GENERATED_PUBKEY})
 endif()
 
-# CONF_FILE points to the KConfig configuration files of the bootloader.
-unset(CONF_DIR)
-foreach(filepath ${CONF_FILE})
-  file(READ ${filepath} temp_text)
-  string(FIND "${temp_text}" ${CONFIG_BOOT_ENCRYPTION_KEY_FILE} match)
-  if(${match} GREATER_EQUAL 0)
-    if(NOT DEFINED CONF_DIR)
-      get_filename_component(CONF_DIR ${filepath} DIRECTORY)
-    else()
-      message(FATAL_ERROR "Encryption key file defined in multiple conf files")
+if(CONFIG_BOOT_ENCRYPTION_KEY_FILE AND NOT CONFIG_BOOT_ENCRYPTION_KEY_FILE STREQUAL "")
+  # CONF_FILE points to the KConfig configuration files of the bootloader.
+  unset(CONF_DIR)
+  foreach(filepath ${CONF_FILE})
+    file(READ ${filepath} temp_text)
+    string(FIND "${temp_text}" ${CONFIG_BOOT_ENCRYPTION_KEY_FILE} match)
+    if(${match} GREATER_EQUAL 0)
+      if(NOT DEFINED CONF_DIR)
+        get_filename_component(CONF_DIR ${filepath} DIRECTORY)
+      else()
+        message(FATAL_ERROR "Encryption key file defined in multiple conf files")
+      endif()
     endif()
-  endif()
-endforeach()
-
+  endforeach()
 
-if(NOT CONFIG_BOOT_ENCRYPTION_KEY_FILE STREQUAL "")
   if(IS_ABSOLUTE ${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
     set(KEY_FILE ${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
   elseif((DEFINED CONF_DIR) AND
@@ -329,7 +328,7 @@ if(NOT CONFIG_BOOT_ENCRYPTION_KEY_FILE STREQUAL "")
   else()
     set(KEY_FILE ${MCUBOOT_DIR}/${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
   endif()
-  message("MCUBoot bootloader key file: ${KEY_FILE}")
+  message("MCUBoot bootloader encryption key file: ${KEY_FILE}")
 
   set(GENERATED_ENCKEY ${ZEPHYR_BINARY_DIR}/autogen-enckey.c)
   add_custom_command(