Add group of certification tools to root BUILD.gn (#5445)

The goal is just to be able to do

  source scripts/activate.sh
  gn gen out/host
  ninja -C out/host certification

on a Raspberry Pi 4 to build the right bits in 2m20s instead of 10
minutes. The group contents can change as tools are added or removed.

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 5d15e38..0dcaacb 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -96,6 +96,7 @@
 
     if (chip_build_tools) {
       deps += [
+        ":certification",
         "${chip_root}/examples/shell/standalone:chip-shell",
         "${chip_root}/src/app/tests/integration:chip-im-initiator",
         "${chip_root}/src/app/tests/integration:chip-im-responder",
@@ -117,6 +118,18 @@
     }
   }
 
+  if (chip_build_tools) {
+    group("certification") {
+      data_deps = [ "${chip_root}/examples/chip-tool" ]
+
+      if (chip_enable_python_modules) {
+        data_deps += [ "${chip_root}/src/controller/python" ]
+      }
+
+      write_runtime_deps = "${root_out_dir}/certification.runtime_deps"
+    }
+  }
+
   group("check") {
     if (chip_link_tests) {
       deps = [ "//src:tests_run" ]