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/chromebook_coral_defconfig b/configs/chromebook_coral_defconfig
index 0cd8f39..c04b0dd 100644
--- a/configs/chromebook_coral_defconfig
+++ b/configs/chromebook_coral_defconfig
@@ -77,6 +77,8 @@
 # CONFIG_SPL_EFI_PARTITION is not set
 CONFIG_OF_SPL_REMOVE_PROPS="clocks clock-names interrupt-parent interrupts linux-name acpi,name acpi,path u-boot,acpi-dsdt-order u-boot,acpi-ssdt-order"
 CONFIG_ENV_OVERWRITE=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="bzImage"
 CONFIG_TFTP_TSIZE=y
 CONFIG_REGMAP=y
 CONFIG_SYSCON=y