pw_rpc: Provide option for disabling the mutex

- Provide a config that disable the global mutex for pw_rpc to simplify
  changing that setting from GN.
- Remove uses of pw_rpc:use_global_mutex since the global mutex is
  enabled by default now.

Change-Id: I80f8db797b8fc1141d85a8970425e22b6aa6a06e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/95844
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Rob Mohr <mohrr@google.com>

pw_rpc/BUILD.gn

@@ -31,13 +31,24 @@ config("public_include_path") {
   visibility = [ ":*" ]
 }
 
+config("disable_global_mutex_config") {
+  defines = [ "PW_RPC_USE_GLOBAL_MUTEX=0" ]
+  visibility = [ ":*" ]
+}
+
+# Set pw_rpc_CONFIG to this to disable the global mutex. If additional options
+# are needed, a config target that sets those can depend on this.
+group("disable_global_mutex") {
+  public_configs = [ ":disable_global_mutex_config" ]
+}
+
 config("global_mutex_config") {
   defines = [ "PW_RPC_USE_GLOBAL_MUTEX=1" ]
   visibility = [ ":*" ]
 }
 
-# Set pw_rpc_CONFIG to this to enable the global mutex. If additional options
-# are needed, a config target that sets those can depend on this.
+# Set pw_rpc_CONFIG to this to always enable the global mutex. The mutex is
+# enabled by default, so this is a no-op.
 group("use_global_mutex") {
   public_configs = [ ":global_mutex_config" ]
 }

pw_system/system_target.gni

@@ -106,8 +106,6 @@ template("pw_system_target") {
     # is added.
     pw_log_BACKEND = dir_pw_log_basic
 
-    pw_rpc_CONFIG = "$dir_pw_rpc:use_global_mutex"
-
     # TODO(amontanez): This should be set to a "$dir_pw_unit_test:rpc_main"
     # when RPC is working.
     pw_unit_test_MAIN = "$dir_pw_unit_test:logging_main"

targets/host/target_toolchains.gni

@@ -55,7 +55,6 @@ _host_common = {
 
   # Configure backend for pw_rpc_system_server.
   pw_rpc_system_server_BACKEND = "$dir_pigweed/targets/host:system_rpc_server"
-  pw_rpc_CONFIG = "$dir_pw_rpc:use_global_mutex"
 
   # Configure backend for trace facade.
   pw_trace_BACKEND = "$dir_pw_trace_tokenized"