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_stratix10_atf_defconfig b/configs/socfpga_stratix10_atf_defconfig
index 60e7750..386a773 100644
--- a/configs/socfpga_stratix10_atf_defconfig
+++ b/configs/socfpga_stratix10_atf_defconfig
@@ -48,6 +48,8 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="kernel.itb"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_DM_SEQ_ALIAS=y
 CONFIG_SPL_ALTERA_SDRAM=y