doc: Explain how to avoid the distro-boot scripts

Now that standard boot is available, mention this in the environment
documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> # Intel Edison
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

doc/usage/environment.rst

@@ -94,7 +94,7 @@ to add environment variables.
 
 Board maintainers are encouraged to migrate to the text-based environment as it
 is easier to maintain. The distro-board script still requires the old-style
-environment but work is underway to address this.
+environments, so use :doc:`../develop/bootstd` instead.
 
 
 List of environment variables