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_ram_defconfig b/configs/at91rm9200ek_ram_defconfig
index 2043923..96a4536 100644
--- a/configs/at91rm9200ek_ram_defconfig
+++ b/configs/at91rm9200ek_ram_defconfig
@@ -5,3 +5,4 @@
 CONFIG_SYS_PROMPT="U-Boot> "
 # CONFIG_CMD_FPGA is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y