cerbero: Remove an outdated build-tools error about bootstrap

The instruction given is no longer valid. `-c config/foo bootstrap`
is a superset of `bootstrap` now.

Also, this error gets hit when we run `fetch-bootstrap` for the first
time.

cerbero/bootstrap/build_tools.py

@@ -89,8 +89,8 @@ class BuildTools (BootstrapperBase, Fetch):
         config.build_tools_cache = self.config.build_tools_cache
         config.external_recipes = self.config.external_recipes
 
-        if config.toolchain_prefix and not os.path.exists(config.toolchain_prefix):
-            raise ConfigurationError(_("Please run bootstrap without any '-c' arguments first to setup build-tools for this machine"))
+        if not os.path.exists(config.toolchain_prefix):
+            os.makedirs(config.toolchain_prefix)
         if not os.path.exists(config.prefix):
             os.makedirs(config.prefix)
         if not os.path.exists(config.sources):