app/prj.conf: add CONFIG_BUILD_OUTPUT_STRIPPED=y

The discussion in
zephyrproject-rtos/zephyr#51954 may take time
and we need reproducibility last year (who doesn't?)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

app/prj.conf

@@ -1,5 +1,14 @@
 CONFIG_SOF=y
 CONFIG_BUILD_OUTPUT_BIN=n
+
+# The additional stripped .elf files are deterministic.
+# This adds very few files in the build directory and has
+# absolutely no other effect whatsoever.
+# See on-going discussion in
+# https://github.com/zephyrproject-rtos/zephyr/pull/51954
+# and usage in xtensa-build-zephyr.py
+CONFIG_BUILD_OUTPUT_STRIPPED=y
+
 CONFIG_HAVE_AGENT=n
 
 CONFIG_LOG=y