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/mpc5121ads_rev2_defconfig b/configs/mpc5121ads_rev2_defconfig
index 4007351..b719f25 100644
--- a/configs/mpc5121ads_rev2_defconfig
+++ b/configs/mpc5121ads_rev2_defconfig
@@ -5,6 +5,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="MPC5121ADS_REV2"
 CONFIG_BOOTDELAY=5
+CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_I2C=y