floss: Refactor build script and merge bootstrapping

Removed the separate bootstrap.py and merged its functionality into
build.py. With all the changes, this is how you do bootstrapping
+ building now:

./build.py --run-bootstrap
./build.py

This changes defaults to something reasonable in the scripts. A few of
the defaults that were changed:

* Set bootstrap dir default to ~/.floss
* Libdir changed to /usr/lib
* Clang made default and --no-clang added instead
* Vendored rust made default and --no-vendored-rust added instead

Bug: 200620285
Tag: #floss
Test: ./build.py --run-bootstrap && ./build.py
Change-Id: I305c6baa8a47e958dfa8e136eb60b08fc742effc