README.falcon: Document environment variables for falcon mode

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

doc/README.falcon

@@ -80,6 +80,15 @@ spl_start_uboot() : required
 		Returns "0" if SPL should start the kernel, "1" if U-Boot
 		must be started.
 
+Environment variables
+---------------------
+
+A board may chose to look at the environment for decisions about falcon
+mode.  In this case the following variables may be supported:
+
+boot_os : 		Set to yes/Yes/true/True/1 to enable booting to OS,
+			any other value to fall back to U-Boot (including
+			unset)
 
 Using spl command
 -----------------