SDK: Also read sdk/build/tools.OS.atree

This allows us to have per-host OS (or OS + arch) atree files in case we
need to get tools which source varies per host OS, example:
- sdk/build/tools.linux.atree
- sdk/build/tools.linux-x86_64.atree
- sdk/build/tools.windows.atree
etc.

Change-Id: I9a1256548345fb866b0750fce6810913413dd946