fastboot: Extract common definitions from USB fastboot

Move FASTBOOT_VERSION to include/fastboot.h so when we merge the UDP code
we only have one definition.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>