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/am57xx_evm_defconfig b/configs/am57xx_evm_defconfig
index c19ec97..2769d35 100644
--- a/configs/am57xx_evm_defconfig
+++ b/configs/am57xx_evm_defconfig
@@ -28,6 +28,7 @@
 CONFIG_ANDROID_AB=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_DMA=y
+CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot.img"
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_OS_BOOT=y