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/omap3_overo_defconfig b/configs/omap3_overo_defconfig
index 4c2a17d..2958e24 100644
--- a/configs/omap3_overo_defconfig
+++ b/configs/omap3_overo_defconfig
@@ -11,3 +11,4 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NFS is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y