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: NTom Rini <trini@ti.com>

README

@@ -2311,6 +2311,12 @@ CBFS (Coreboot Filesystem) support
 		- CONFIG_SYS_VENDOR
 		- CONFIG_SYS_SOC
 
+		CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
+
+		Define this in order to add variables describing certain
+		run-time determined information about the hardware to the
+		environment.  These will be named board_name, board_rev.
+
 - DataFlash Support:
 		CONFIG_HAS_DATAFLASH