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/syzygy_hub_defconfig b/configs/syzygy_hub_defconfig
index 4b4f692..b199084 100644
--- a/configs/syzygy_hub_defconfig
+++ b/configs/syzygy_hub_defconfig
@@ -26,6 +26,7 @@
 CONFIG_USE_PREBOOT=y
 CONFIG_CLOCKS=y
 CONFIG_SPL_STACK_R=y
+CONFIG_SPL_FS_LOAD_ARGS_NAME="system.dtb"
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x0