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/a4m2k_defconfig b/configs/a4m2k_defconfig
index 22db933..5d52092 100644
--- a/configs/a4m2k_defconfig
+++ b/configs/a4m2k_defconfig
@@ -5,3 +5,4 @@
 CONFIG_SYS_EXTRA_OPTIONS="A4M2K"
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_LIB_RAND=y
+CONFIG_OF_LIBFDT=y