tools/bootconfig: Suppress non-error messages

Suppress non-error messages when applying new bootconfig
to initrd image. To enable it, replace printf for error
message with pr_err() macro.
This also adds a testcase for this fix.

Link: http://lkml.kernel.org/r/158125351377.16911.13283712972275131160.stgit@devnote2



Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>