Convert CONFIG_SPL_FS_LOAD_PAYLOAD_NAME et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_FS_LOAD_ARGS_NAME
   CONFIG_SPL_FS_LOAD_KERNEL_NAME
   CONFIG_SPL_FS_LOAD_PAYLOAD_NAME

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mx6ul_9x9_evk_defconfig b/configs/mx6ul_9x9_evk_defconfig
index d2ef51e..2f46b68 100644
--- a/configs/mx6ul_9x9_evk_defconfig
+++ b/configs/mx6ul_9x9_evk_defconfig
@@ -28,6 +28,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_FS_EXT4=y
+CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot-dtb.img"
 CONFIG_SPL_I2C=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y