tools/bootconfig: Improve portability

Since 'stat' command and 'truncate' command are GNU extension,
use 'wc' and 'dd' commands instead.

Link: https://lore.kernel.org/all/175211424184.2591046.3523297993175066026.stgit@mhiramat.tok.corp.google.com/



Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>