Currently it is possible to cancel loadx and loady commands by pressing
CTRL+X (CAN character) at least 3 times quickly.

All other U-Boot commands, including loadb and loads can be cancelled by
CTRL+C. So allow it also in xyz-modem code used by loadx and loady
commands. Implement it by handling CTRL+C (ETX character) in the same way
as CTRL+X (CAN character).

Due to how x/y-modem protocol works, it is required to press
CTRL+C or CTRL+X at least 3 times quickly.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>