Define docs target as a toolchain

This change creates a toolchain that configures the docs target and adds
it to the default build.

Change-Id: Ib2a8387253eef809f94411780174377e0d23e1a9
diff --git a/pw_sys_io_baremetal_stm32f429/BUILD.gn b/pw_sys_io_baremetal_stm32f429/BUILD.gn
index f1d9466..cc232fc 100644
--- a/pw_sys_io_baremetal_stm32f429/BUILD.gn
+++ b/pw_sys_io_baremetal_stm32f429/BUILD.gn
@@ -15,25 +15,19 @@
 # gn-format disable
 import("//build_overrides/pigweed.gni")
 
-import("$dir_pigweed/legacy_target.gni")
 import("$dir_pw_build/target_types.gni")
 import("$dir_pw_docgen/docs.gni")
-
-# This if statement allows docs to always build even if the target isn't
-# compatible with this backend.
-if (dir_pw_sys_io_backend == dir_pw_sys_io_baremetal_stm32f429) {
-  pw_source_set("pw_sys_io_baremetal_stm32f429") {
-    public_deps = [ "$dir_pw_boot_armv7m" ]
-    deps = [
-      "$dir_pw_preprocessor",
-      "$dir_pw_sys_io:default_putget_bytes",
-      "$dir_pw_sys_io:facade",
-    ]
-    sources = [
-      "early_boot.c",
-      "sys_io_baremetal.cc",
-    ]
-  }
+pw_source_set("pw_sys_io_baremetal_stm32f429") {
+  public_deps = [ "$dir_pw_boot_armv7m" ]
+  deps = [
+    "$dir_pw_preprocessor",
+    "$dir_pw_sys_io:default_putget_bytes",
+    "$dir_pw_sys_io:facade",
+  ]
+  sources = [
+    "early_boot.c",
+    "sys_io_baremetal.cc",
+  ]
 }
 
 pw_doc_group("docs") {