cmake: kconfig: skip the Kconfig checksum recalculation when it is unused

The loop that recalculates the checksum after kconfig.py runs is only
consumed inside if(CREATE_NEW_DOTCONFIG). On any configure that does not
regenerate .config its result is discarded, so hashing all 5001 parsed
Kconfig sources is pure waste, measured in isolation at 168 ms.

Move the loop into the block that consumes it. The file(STRINGS) read
above stays where it is, as it also feeds the CMAKE_CONFIGURE_DEPENDS
registration, which must keep running unconditionally.

Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

cmake/modules/kconfig.cmake

@@ -435,14 +435,6 @@ endif()
 # Read out the list of 'Kconfig' sources that were used by the engine.
 file(STRINGS ${PARSED_KCONFIG_SOURCES_TXT} parsed_kconfig_sources_list ENCODING UTF-8)
 
-# Recalculate the Kconfig files' checksum, since the list of files may have
-# changed.
-set(merge_kconfig_checksum "")
-foreach(f ${parsed_kconfig_sources_list})
-  file(MD5 ${f} checksum)
-  string(APPEND merge_kconfig_checksum "${checksum}")
-endforeach()
-
 # Force CMAKE configure when the Kconfig sources or configuration files changes.
 set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS
   ${merge_config_files}
@@ -451,6 +443,14 @@ set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS
 )
 
 if(CREATE_NEW_DOTCONFIG)
+  # Recalculate the Kconfig files' checksum, since the list of files may have
+  # changed.
+  set(merge_kconfig_checksum "")
+  foreach(f ${parsed_kconfig_sources_list})
+    file(MD5 ${f} checksum)
+    string(APPEND merge_kconfig_checksum "${checksum}")
+  endforeach()
+
   # Write the new configuration fragment checksum. Only do this if kconfig.py
   # succeeds, to avoid marking zephyr/.config as up-to-date when it hasn't been
   # regenerated.