tools/bootconfig: Fix errno reference after printf()

Fix not to refer the errno variable as the result of previous libc
functions after printf() because printf() can change the errno.

Link: https://lkml.kernel.org/r/160576520243.320071.51093664672431249.stgit@devnote2

Fixes: 85c46b78

 ("bootconfig: Add bootconfig magic word for indicating bootconfig explicitly")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>