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/am64x_evm_a53_defconfig b/configs/am64x_evm_a53_defconfig
index 489a750..ed603a1 100644
--- a/configs/am64x_evm_a53_defconfig
+++ b/configs/am64x_evm_a53_defconfig
@@ -38,6 +38,7 @@
 CONFIG_SPL_DMA=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_ETH=y
+CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot.img"
 CONFIG_SPL_I2C=y
 CONFIG_SPL_DM_MAILBOX=y
 CONFIG_SPL_DM_SPI_FLASH=y