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/MPC8536DS_SPIFLASH_defconfig b/configs/MPC8536DS_SPIFLASH_defconfig
index 83eb24d..7bb7087 100644
--- a/configs/MPC8536DS_SPIFLASH_defconfig
+++ b/configs/MPC8536DS_SPIFLASH_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="SPIFLASH"
 CONFIG_BOOTDELAY=10
 # CONFIG_CONSOLE_MUX is not set
+CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y