x86: Add CONFIG_DELAY_ENVIRONMENT to delay environment loading

This option delays loading of the environment until later, so that only the
default environment will be available to U-Boot.

This can address the security risk of untrusted data being used during boot.

When CONFIG_DELAY_ENVIRONMENT is defined, it is convenient to have a
run-time way of enabling loadinlg of the environment. Add this to the
fdt as /config/delay-environment.

Note: This patch depends on http://patchwork.ozlabs.org/patch/194342/



Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>