board: puma: Get bl31.bin via BL31 and rk3399m0.bin via PMUM0

Right now puma rk3399 board need to copy bl31-rk3399.bin and
rk3399m0.bin into u-boot source directory to make use of building
u-boot.itb.

So, add environment variable
- BL31 for bl31.bin (instead of bl31-rk3399.bin to compatible with other
  platform BL31 env)
- PMUM0 for rk3399m0.bin

If the builds are not exporting BL31, PMUM0 env, the fit_spl_atf.sh will
notify with warning about which document to refer for more information
like this:

 WARNING: BL31 file bl31.bin NOT found, resulting binary is non-functional
 Please read Building section in doc/README.rockchip
 WARNING: PMUM0 file rk3399m0.bin NOT found, resulting binary is non-functional
 Please read Building section in doc/README.rockchip

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/configs/puma-rk3399_defconfig b/configs/puma-rk3399_defconfig
index 03f0bfd..ebc78fb 100644
--- a/configs/puma-rk3399_defconfig
+++ b/configs/puma-rk3399_defconfig
@@ -15,7 +15,7 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
 CONFIG_DEBUG_UART=y
-CONFIG_SPL_FIT_SOURCE="board/theobroma-systems/puma_rk3399/fit_spl_atf.its"
+CONFIG_SPL_FIT_GENERATOR="board/theobroma-systems/puma_rk3399/fit_spl_atf.sh"
 CONFIG_DEFAULT_FDT_FILE="rockchip/rk3399-puma-haikou.dtb"
 CONFIG_MISC_INIT_R=y
 # CONFIG_DISPLAY_CPUINFO is not set