toolchain/zephyr: invoke ar with -D for deterministic .a files

Quoting GNU ar man/info page:

  'D'
     Operate in _deterministic_ mode.  When adding files and the archive
     index use zero for UIDs, GIDs, timestamps, and use consistent file
     modes for all files.  When this option is used, if 'ar' is used
     with identical options and identical input files, multiple runs
     will create identical output files regardless of the input files'
     owners, groups, file modes, or modification times.

     If 'binutils' was configured with
     '--enable-deterministic-archives', then this mode is on by default.
     It can be disabled with the 'U' modifier, below.

Signed-off-by: Marc Herbert <[email protected]>

cmake/toolchain/zephyr/target.cmake

@@ -1,3 +1,34 @@
 # SPDX-License-Identifier: Apache-2.0
 
 include(${ZEPHYR_BASE}/cmake/toolchain/zephyr/${SDK_VERSION}/target.cmake)
+
+
+# For toolchains without a deterministic 'ar' option, see
+# less ideal but tested "strip-nondeterminism" alternative at
+#  https://github.com/marc-hb/zephyr/commits/strip-nondeterminism
+
+# TODO: build future versions of the Zephyr SDK with
+#      --enable-deterministic-archives?
+
+# Ideally we'd want to _append_ -D to the list of flags and operations
+# that cmake already uses by default. However as of version 3.14 cmake
+# doesn't support addition, one can only replace the complete AR command.
+
+# To see what the default flags are and find where they're defined
+# in cmake's code:
+# - comment out the end of the foreach line like this:
+#      foreach(lang ) # ASM C CXX
+# - build any sample with:
+#       2>cmake.log  cmake --trace-expand ...
+# - Search cmake.log for:
+#      CMAKE_.*_ARCHIVE and CMAKE_.*CREATE_STATIC
+
+# CMake's documentation is at node: CMAKE_LANG_CREATE_STATIC_LIBRARY
+
+# At least one .a file needs 'ASM': arch/xtensa/core/startup/ because
+# there's no C file there.
+foreach(lang ASM C CXX)
+  # GNU ar always updates the index: no need for CMAKE_RANLIB
+  SET(CMAKE_${lang}_CREATE_STATIC_LIBRARY
+      "<CMAKE_AR> qcD <TARGET> <LINK_FLAGS> <OBJECTS>")
+endforeach()