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/socrates_defconfig b/configs/socrates_defconfig
index 19cc388..39e7ffb 100644
--- a/configs/socrates_defconfig
+++ b/configs/socrates_defconfig
@@ -4,3 +4,4 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NFS is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y