Function _start

pub unsafe extern "C" fn _start()

This assembly does three functions:

1. It initializes the stack pointer, the frame pointer (needed for closures to work in start_rust) and the global pointer.
2. It initializes the .bss and .data RAM segments. This must be done before any Rust code runs. See https://github.com/tock/tock/issues/2222 for more information.
3. Finally it calls main(), the main entry point for Tock boards.