efi_loader: initrddump: drain input before prompt

Up to now the initrddump.efi application has drained the input after
showing the prompt. This works for humans but leads to problems when
automating testing. If the input is drained, this should be done before
showing the prompt.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>