tools: include u-boot version of sha256.h

When building tools the u-boot specific sha256.h is required, but the
host version of sha256.h is used when present. This leads to build errors
on FreeBSD which does have a system sha256.h include. Like libfdt_env.h
explicitly include u-boot's sha256.h.

cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Acked-by: Simon Glass <sjg@chromium.org>