pw_system: Add target_hooks porting layer

Instead of hard-coding against FreeRTOS, uses a porting layer defined in
pw_system/target_hooks.h to enable other schedulers.

Change-Id: Idc7446f27ae1d0d89f9cb02a12f7e76d4b8dfcbd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/74860
Reviewed-by: Carlos Chinchilla <cachinchilla@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/pw_system/backend.gni b/pw_system/backend.gni
new file mode 100644
index 000000000..ec8a7fcab
--- /dev/null
+++ b/pw_system/backend.gni
@@ -0,0 +1,21 @@
+# Copyright 2021 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+import("//build_overrides/pigweed.gni")
+
+declare_args() {
+  # The pw_system backend that provides thread options for the appropriate
+  # scheduler.
+  pw_system_BACKEND = ""
+}