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/Makefile b/Makefile
index db81531..f3857ab 100644
--- a/Makefile
+++ b/Makefile
@@ -1848,7 +1848,7 @@
 		-o -name 'dsdt.aml' -o -name 'dsdt.asl.tmp' -o -name 'dsdt.c' \
 		-o -name '*.efi' -o -name '*.gcno' -o -name '*.so' \) \
 		-type f -print | xargs rm -f \
-		bl31_*.bin image.map
+		bl31.c bl31.elf bl31_*.bin image.map
 
 # mrproper - Delete all generated files, including .config
 #