Roll buildroot to ef514ca5144ba0a4ce5e7075c325d24cd6da73a4

In flutter#51258, all existing build_overrides from the buildroot
were copied into the engine.

In flutter/buildroot#832, all buildroot build_overrides were converted
into simple forwarding files that point to the ones in the engine such
that we have a seamless path to eliminating the buildroot, while
allowing third-party dependencies to continue to hardcode
`//build_overrides/foo.gni`.

This rolls the buildroot to the engine and switches everthing over to
the new engine-based build_overrides.

Issue: flutter/flutter#144790
Part of: flutter/flutter#67373

DEPS

@@ -283,7 +283,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + '31f887a3ba13087f1bc64bd2a1736329b5f84201',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + 'ef514ca5144ba0a4ce5e7075c325d24cd6da73a4',
 
   'src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',