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/mx6sabreauto_defconfig b/configs/mx6sabreauto_defconfig
index f96bbcd..3d2e906 100644
--- a/configs/mx6sabreauto_defconfig
+++ b/configs/mx6sabreauto_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SPL_LEGACY_IMAGE_FORMAT=y
 CONFIG_SPL_FIT_IMAGE_TINY=y
 CONFIG_SPL_FS_EXT4=y
+CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot-dtb.img"
 CONFIG_SPL_I2C=y
 CONFIG_SPL_USB_HOST=y
 CONFIG_SPL_USB_GADGET=y