Now that am335x_evm boots OK on the Beaglebone black, drop the latter and update the docs to cover the change. Also add a few updates about 'make fit' and drop the note about the security review, as U-Boot's verified boot has had quite extensive review now. Signed-off-by: Simon Glass <sjg@chromium.org> Reviewed-by: Tom Rini <trini@konsulko.com>