Move CONFIG_OF_LIBFDT to Kconfig

Move this option to Kconfig and tidy up existing boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ot1200_defconfig b/configs/ot1200_defconfig
index a1ecf06..5e8671f 100644
--- a/configs/ot1200_defconfig
+++ b/configs/ot1200_defconfig
@@ -10,3 +10,4 @@
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_SPI_FLASH_SST=y
 CONFIG_SPI_FLASH_WINBOND=y
+CONFIG_OF_LIBFDT=y