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_SDCARD_defconfig b/configs/P4080DS_SDCARD_defconfig
index 25f7861..b9dbb7b 100644
--- a/configs/P4080DS_SDCARD_defconfig
+++ b/configs/P4080DS_SDCARD_defconfig
@@ -38,6 +38,8 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_DM=y
 CONFIG_FSL_CAAM=y
 CONFIG_DDR_ECC=y