tools: Allow crc8 to be used

This patch enables crc8 to be used from within the tools directory using
u-boot/crc.h.

Signed-off-by: Olliver Schinagl <o.schinagl@ultimaker.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>