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/mx6dlarm2_lpddr2_defconfig b/configs/mx6dlarm2_lpddr2_defconfig
index 9ee4ee7..01a682f 100644
--- a/configs/mx6dlarm2_lpddr2_defconfig
+++ b/configs/mx6dlarm2_lpddr2_defconfig
@@ -6,3 +6,4 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y