targets/stm32f769i_disc0: Disable pw_rpc mutex since there is no RTOS

Disable the pw_rpc mutex since there is no RTOS. An upcoming Pigweed CL
will add a pw_rpc dependency on pw_thread, unless the mutex is disabled.

Change-Id: I6331815180855c336921ea8f5f2064235348d5f2
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/126704
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
diff --git a/targets/stm32f769i_disc0/pw_target_toolchains.gni b/targets/stm32f769i_disc0/pw_target_toolchains.gni
index 9080da1..d7d79b3 100644
--- a/targets/stm32f769i_disc0/pw_target_toolchains.gni
+++ b/targets/stm32f769i_disc0/pw_target_toolchains.gni
@@ -57,6 +57,7 @@
   pw_sync_INTERRUPT_SPIN_LOCK_BACKEND =
       "$dir_pw_sync_baremetal:interrupt_spin_lock"
   pw_sync_MUTEX_BACKEND = "$dir_pw_sync_baremetal:mutex"
+  pw_rpc_CONFIG = "$dir_pw_rpc:disable_global_mutex"
   pw_log_BACKEND = dir_pw_log_basic
   pw_sys_io_BACKEND = dir_pw_sys_io_baremetal_stm32f769
   pw_rpc_system_server_BACKEND = "$dir_pw_hdlc:hdlc_sys_io_system_server"