Convert CONFIG_BOOTFILE to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTFILE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/socfpga_n5x_atf_defconfig b/configs/socfpga_n5x_atf_defconfig
index 31346fc..d458818 100644
--- a/configs/socfpga_n5x_atf_defconfig
+++ b/configs/socfpga_n5x_atf_defconfig
@@ -47,6 +47,8 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_ENV_IS_IN_MMC=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="kernel.itb"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_DM_SEQ_ALIAS=y
 CONFIG_DWAPB_GPIO=y