Add build.py to simplify building locally

This adds a script that can build system/bt locally on your Linux
machine. Please see required folder layout in the script on how to
prepare your environment and check the help text on how to use it.

In its simplest form (for example):
  ./build.py --out ../build --platform-dir ../staging --clang

This will build all of systembt in ../build using the ../staging
directory as your source directory and will use the clang compiler to do
the build.

Bug: 179821440
Tag: #floss
Test: atest --host bluetooth_test_gd

Change-Id: I9c6a9fa30235b2fc661a0fe327154f18a03ca026