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/openpiton_riscv64_spl_defconfig b/configs/openpiton_riscv64_spl_defconfig
index a5c01d3..6e4e454 100644
--- a/configs/openpiton_riscv64_spl_defconfig
+++ b/configs/openpiton_riscv64_spl_defconfig
@@ -25,6 +25,7 @@
 # CONFIG_SPL_BANNER_PRINT is not set
 CONFIG_SPL_CPU=y
 CONFIG_SPL_FS_EXT4=y
+CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="boot/fw_payload.bin"
 CONFIG_SPL_RTC=y
 CONFIG_SYS_PROMPT="openpiton$ "
 CONFIG_SYS_CBSIZE=256