Added convenience mode to build toolchain using a external dir with  a checked-out pnacl-sfi branch. This comes in handy for testing.
Added convenience mode to build toolchain using a external dir with 
a checked-out pnacl-sfi branch.
This comes in handy for testing.

Review URL: http://codereview.chromium.org/2854028

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@2587 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client