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/nitrogen6s1g_defconfig b/configs/nitrogen6s1g_defconfig
index 1572b9b..7e2eb86 100644
--- a/configs/nitrogen6s1g_defconfig
+++ b/configs/nitrogen6s1g_defconfig
@@ -6,6 +6,7 @@
 CONFIG_BOOTDELAY=3
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 # CONFIG_SYS_STDIO_DEREGISTER is not set
+CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set