Convert CONFIG_BOARD_EARLY_INIT_F to Kconfig

This converts the following to Kconfig:
   CONFIG_BOARD_EARLY_INIT_F

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx6slevk_spl_defconfig b/configs/mx6slevk_spl_defconfig
index 4dd2484..29e179d 100644
--- a/configs/mx6slevk_spl_defconfig
+++ b/configs/mx6slevk_spl_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,SPL,MX6SL"
 CONFIG_BOOTDELAY=3
+CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y