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/at91rm9200ek_defconfig b/configs/at91rm9200ek_defconfig
index e743b53..7c99aa0 100644
--- a/configs/at91rm9200ek_defconfig
+++ b/configs/at91rm9200ek_defconfig
@@ -4,3 +4,4 @@
 CONFIG_SYS_PROMPT="U-Boot> "
 # CONFIG_CMD_FPGA is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y