README: Document CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG

This option is intended to be set by boards which will set the
board_name and board_rev environment variables.  These are to be used
when the U-Boot binary can support more than one board type at run-time
and the user needs an easy way (for example for scripting to determine
what device tree to load) to determine what board they are on.

Signed-off-by: Tom Rini <trini@ti.com>