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>
1 file changed