tools/bootconfig: Replace some echo with printf for more portability

Since echo is not portable, use printf instead. This fixes a wrong
test result formatting in some environment. (showing "\t\t[OK]")
Also this uses printf command for generating test data instead of
echo.

Link: https://lore.kernel.org/all/175211424942.2591046.5439447789303314213.stgit@mhiramat.tok.corp.google.com/



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