efi_selftest: BitBlt test

The BitBlt test leaves the serial console output in disarray.

* Call ClearScreen() where needed.
* Test CheckEvent() for EFI_SIMPLE_TEXT_INPUT_PROTOCOL.WaitForKey
  via adding navigation keys
* Correct timer comment

For testing on the sandbox:

    CONFIG_CMD_BOOTEFI_SELFTEST=y
    CONFIG_CONSOLE_TRUETYPE=n
    $ ./u-boot -T -l
    => setenv efi_selftest block image transfer
    => bootefi selftest

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