rockchip: rk3399: Get bl31.elf via BL31

Right now rockchip platform need to copy bl31.elf into u-boot
source directory to make use of building u-boot.itb.

So, add environment variable BL31 like Allwinner SoC so-that the
bl31.elf would available via BL31.

If the builds are not exporting BL31 env, the make_fit_atf.py
explicitly create dummy bl31.elf in u-boot root directory to
satisfy travis builds and it will show the warning on console as

 WARNING: BL31 file bl31.elf NOT found, resulting binary is non-functional
 WARNING: Please read Building section in doc/README.rockchip

Note, that the dummy bl31 files were created during not exporting
BL31 case would be removed via clean target in Makefile.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/doc/README.rockchip b/doc/README.rockchip
index 11ab74a..4efd9c1 100644
--- a/doc/README.rockchip
+++ b/doc/README.rockchip
@@ -150,8 +150,8 @@
 	=> make realclean
 	=> make CROSS_COMPILE=aarch64-linux-gnu- PLAT=rk3399
 
-	(copy bl31.elf into U-Boot root dir)
-	=> cp build/rk3399/release/bl31/bl31.elf /path/to/u-boot
+	(export bl31.elf)
+	=> export BL31=/path/to/arm-trusted-firmware/build/rk3399/release/bl31/bl31.elf
 
    - Compile PMU M0 firmware