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/P4080DS_defconfig b/configs/P4080DS_defconfig
index e3c41c3..1d0c710 100644
--- a/configs/P4080DS_defconfig
+++ b/configs/P4080DS_defconfig
@@ -34,6 +34,8 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xEFF20000
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_DM=y
 CONFIG_FSL_CAAM=y
 CONFIG_DDR_ECC=y