README.falcon: Note how we determine if we can boot the OS or not

Reviewed-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Tom Rini <trini@ti.com>

doc/README.falcon

@@ -41,6 +41,8 @@ file (CONFIG_CMD_SPL_NAND_OFS for NAND).
 
 3. Boot the board into Falcon Mode. SPL will load the kernel and copy
 the parameters which are saved in the persistent area to the required address.
+If a valid uImage is not found at the defined location, U-Boot will be
+booted instead.
 
 It is required to implement a custom mechanism to select if SPL loads U-Boot
 or another image.