floss: Add bootstrap script to simplify setup

To simplify setting up the Linux build, add a bootstrap script that will
set up the output and staging directories automatically. The script also
checks if you have all requisite packages installed to build
successfully or it will generate an apt-get install command for you with
the missing packages. Also checks rust packages.

Bug: 193585779
Test: Ran script
Tag: #floss
Change-Id: Iede4f442af2bc8028623cbe808afd99951cf723c