serial: Add EFI stdio driver ...

This adds a driver which uses the EFI stdin/stdout interfaces to
implement a barebox console. Keyboard input should be fairly complete,
but not all vt100 needed by barebox work properly. The clear-to-eol
escape is missing causing garbled output in the editor.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>