fastboot: Refactor fastboot_okay/fail to take response

Add the response string as a parameter to fastboot_okay/fail, instead
of modifying a global, to match the contract expected by the AOSP
U-Boot code.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>