FIT: Rename FIT_DISABLE_SHA256 to FIT_ENABLE_SHA256_SUPPORT

We rename CONFIG_FIT_DISABLE_SHA256 to CONFIG_FIT_ENABLE_SHA256_SUPPORT which
is enabled by default and now a positive option.  Convert the handful of boards
that were disabling it before to save space.

Cc: Dirk Eibach <eibach@gdsys.de>
Cc: Lukasz Dalek <luk0104@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>