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_spinor_defconfig b/configs/mx6slevk_spinor_defconfig
index 00dbebd..7c0a3a8 100644
--- a/configs/mx6slevk_spinor_defconfig
+++ b/configs/mx6slevk_spinor_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_MX6SLEVK=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6slevk/imximage.cfg,MX6SL,SYS_BOOT_SPINOR"
 CONFIG_BOOTDELAY=3
+CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set