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/at91sam9x5ek_mmc_defconfig b/configs/at91sam9x5ek_mmc_defconfig
index 29fdf31..b782e25 100644
--- a/configs/at91sam9x5ek_mmc_defconfig
+++ b/configs/at91sam9x5ek_mmc_defconfig
@@ -10,3 +10,4 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y
+CONFIG_OF_LIBFDT=y