download/coreboot: minor code cleanup

i'm pretty much finished now

there might be a few more changes later,
like stricter error handling, more verbose
error messages, etc

right now, it relies on -e to kill lbmk
on error, and uses the exit command

another planned change it to support
other upstreams besides coreboot.org,
such as the dasharo codebase

the latter is *why* i refactored this
download script, for asus kgped-d16