kconfig: echo stdin to stdout if either is redirected

If stdio is not tty, conf_askvalue() puts additional new line to
prevent prompts from being concatenated into a single line.  This
care is missing in conf_choice(), so a 'choice' prompt and the next
prompt are shown in the same line.

Move the code into xfgets() to cater to all cases.  To improve this
more, let's echo stdin to stdout.  This clarifies what keys were
input from stdio and the stdout looks like as if it were from tty.

I removed the isatty(2) check since stderr is unrelated here.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>