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/riotboard_defconfig b/configs/riotboard_defconfig
index 91be32c..d00666b 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -7,6 +7,7 @@
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
+CONFIG_BOARD_EARLY_INIT_F=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y