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/M5235EVB_defconfig b/configs/M5235EVB_defconfig
index 8a12e4b..e2f7839 100644
--- a/configs/M5235EVB_defconfig
+++ b/configs/M5235EVB_defconfig
@@ -24,6 +24,8 @@
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xFFE04000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="u-boot.bin"
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_FSL=y
 CONFIG_SYS_FSL_I2C_OFFSET=0x300