fastboot: Correct fastboot_fail and fastboot_okay strings

If the string is copied without NULL termination using strncpy(),
then strncat() on the next line, may concatenate the string after
some stale (or random) data, if the response string was not
zero-initialized.

Signed-off-by: Dileep Katta <dileep.katta@linaro.org>
Reviewed-by: Steve Rae <srae@broadcom.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>