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

resources/scripts/download/coreboot

@@ -38,12 +38,9 @@ main()
 	elif [ $# -eq 1 ] && [ "$1" = "--list-boards" ] ; then
 		list_supported_boards
 		exit 0
-	fi
-
+	else
 		fetch_coreboot_trees $@
-
-	printf "\n\n"
-	exit 0
+	fi
 }
 
 fetch_coreboot_trees()