pw_system: Make RPC server a facade

Makes pw_system's RPC server a public facade so users can:
 1. Access the RPC server to register services.
 2. Provide a custom server implementation with more channels and
    different transports.

Also adds some real docs to pw_system.

Change-Id: I789089573359061c5f3254575e540c27a63deed3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/88364
Reviewed-by: Carlos Chinchilla <cachinchilla@google.com>
Reviewed-by: Tom Craig <tommycraig@gmail.com>
Reviewed-by: Chad Norvell <chadnorvell@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/pw_system/backend.gni b/pw_system/backend.gni
index 727004ef0..3b4d470df 100644
--- a/pw_system/backend.gni
+++ b/pw_system/backend.gni
@@ -17,8 +17,20 @@ import("//build_overrides/pigweed.gni")
 declare_args() {
   # The pw_system backend that provides thread options for the appropriate
   # scheduler.
+  #
+  # There's no default backend as this is target/os specific. pw_system_target
+  # can automatically configure this for your project.
   pw_system_TARGET_HOOKS_BACKEND = ""
 
   # The pw_system backend that provides read/write streams for RPC and logging.
+  #
+  # There's no default backend as this is target specific. pw_system_target
+  # can automatically configure this for your project.
   pw_system_IO_BACKEND = ""
+
+  # The pw_system backend that provides the system RPC server.
+  #
+  # This defaults to a single-channel HDLC server provided by pw_system
+  # when using a pw_system_target.
+  pw_system_RPC_SERVER_BACKEND = ""
 }