Convert CONFIG_SPL_BOARD_INIT to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_BOARD_INIT

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
[trini: Update the Kconfig logic]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 3061e5a..cdeb30c 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -21,6 +21,7 @@
 CONFIG_CONSOLE_RECORD_OUT_SIZE=0x1000
 CONFIG_SILENT_CONSOLE=y
 CONFIG_SPL=y
+CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y