[Silabs] Add user configurable paths for board configuration files (#28019)

* Add option of user to configure silabs_pre_gen or slc_gen paths

* slc_gen_path can be in efr32_sdk.gni

* Fix pathing issue, use the active python3 installation on any env. Make sure slc has a configured and trusted gsdk set
diff --git a/examples/smoke-co-alarm-app/silabs/BUILD.gn b/examples/smoke-co-alarm-app/silabs/BUILD.gn
index d4ebe73..cafd4b6 100644
--- a/examples/smoke-co-alarm-app/silabs/BUILD.gn
+++ b/examples/smoke-co-alarm-app/silabs/BUILD.gn
@@ -60,6 +60,7 @@
                       "${use_wstk_leds}",
                       "${use_external_flash}",
                       "${silabs_mcu}",
+                      rebase_path(slc_gen_path),
                     ],
                     "list lines"))
 }