tools: kwboot: properly quit when read() returns 0

When kwboot is attached to a terminal which disappears such as one
connected via an unplugged USB cable, read() returns 0, making kwboot
loop until a key is pressed in the terminal. The only case where read()
may return 0 here is when the terminal is closed anyway, so let's
properly handle this one and report is similar to other errors.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Stefan Roese <sr@denx.de>